Failed to create second cluster in shared vnet, below error is thrown out during creating network infrastructure when creating 2nd cluster, installer timed out and exited.
==============
07-23 14:09:27.315  level=info msg=Waiting up to 15m0s (until 6:24AM UTC) for network infrastructure to become ready...
...
07-23 14:16:14.900  level=debug msg=	failed to reconcile cluster services: failed to reconcile AzureCluster service loadbalancers: failed to create or update resource jima0723b-1-x6vpp-rg/jima0723b-1-x6vpp-internal (service: loadbalancers): PUT https://management.azure.com/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-1-x6vpp-rg/providers/Microsoft.Network/loadBalancers/jima0723b-1-x6vpp-internal
07-23 14:16:14.900  level=debug msg=	--------------------------------------------------------------------------------
07-23 14:16:14.901  level=debug msg=	RESPONSE 400: 400 Bad Request
07-23 14:16:14.901  level=debug msg=	ERROR CODE: PrivateIPAddressIsAllocated
07-23 14:16:14.901  level=debug msg=	--------------------------------------------------------------------------------
07-23 14:16:14.901  level=debug msg=	{
07-23 14:16:14.901  level=debug msg=	  "error": {
07-23 14:16:14.901  level=debug msg=	    "code": "PrivateIPAddressIsAllocated",
07-23 14:16:14.901  level=debug msg=	    "message": "IP configuration /subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-1-x6vpp-rg/providers/Microsoft.Network/loadBalancers/jima0723b-1-x6vpp-internal/frontendIPConfigurations/jima0723b-1-x6vpp-internal-frontEnd is using the private IP address 10.0.0.100 which is already allocated to resource /subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/frontendIPConfigurations/jima0723b-49hnw-internal-frontEnd.",
07-23 14:16:14.902  level=debug msg=	    "details": []
07-23 14:16:14.902  level=debug msg=	  }
07-23 14:16:14.902  level=debug msg=	}
07-23 14:16:14.902  level=debug msg=	--------------------------------------------------------------------------------

Install-config for 1st cluster:
=========
metadata:
  name: jima0723b
platform:
  azure:
    region: eastus
    baseDomainResourceGroupName: os4-common
    networkResourceGroupName: jima0723b-rg
    virtualNetwork: jima0723b-vnet
    controlPlaneSubnet: jima0723b-master-subnet
    computeSubnet: jima0723b-worker-subnet
publish: External

Install-config for 2nd cluster:
========
metadata:
  name: jima0723b-1
platform:
  azure:
    region: eastus
    baseDomainResourceGroupName: os4-common
    networkResourceGroupName: jima0723b-rg
    virtualNetwork: jima0723b-vnet
    controlPlaneSubnet: jima0723b-master-subnet
    computeSubnet: jima0723b-worker-subnet
publish: External

shared master subnet/worker subnet:
$ az network vnet subnet list -g jima0723b-rg --vnet-name jima0723b-vnet -otable
AddressPrefix    Name                     PrivateEndpointNetworkPolicies    PrivateLinkServiceNetworkPolicies    ProvisioningState    ResourceGroup
---------------  -----------------------  --------------------------------  -----------------------------------  -------------------  ---------------
10.0.0.0/24      jima0723b-master-subnet  Disabled                          Enabled                              Succeeded            jima0723b-rg
10.0.1.0/24      jima0723b-worker-subnet  Disabled                          Enabled                              Succeeded            jima0723b-rg

internal lb frontedIPConfiguration on 1st cluster:
$ az network lb show -n jima0723b-49hnw-internal -g jima0723b-49hnw-rg --query 'frontendIPConfigurations'
[
  {
    "etag": "W/\"7a7531ca-fb02-48d0-b9a6-d3fb49e1a416\"",
    "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/frontendIPConfigurations/jima0723b-49hnw-internal-frontEnd",
    "inboundNatRules": [
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/inboundNatRules/jima0723b-49hnw-master-0",
        "resourceGroup": "jima0723b-49hnw-rg"
      },
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/inboundNatRules/jima0723b-49hnw-master-1",
        "resourceGroup": "jima0723b-49hnw-rg"
      },
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/inboundNatRules/jima0723b-49hnw-master-2",
        "resourceGroup": "jima0723b-49hnw-rg"
      }
    ],
    "loadBalancingRules": [
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/loadBalancingRules/LBRuleHTTPS",
        "resourceGroup": "jima0723b-49hnw-rg"
      },
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-49hnw-rg/providers/Microsoft.Network/loadBalancers/jima0723b-49hnw-internal/loadBalancingRules/sint-v4",
        "resourceGroup": "jima0723b-49hnw-rg"
      }
    ],
    "name": "jima0723b-49hnw-internal-frontEnd",
    "privateIPAddress": "10.0.0.100",
    "privateIPAddressVersion": "IPv4",
    "privateIPAllocationMethod": "Static",
    "provisioningState": "Succeeded",
    "resourceGroup": "jima0723b-49hnw-rg",
    "subnet": {
      "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/jima0723b-rg/providers/Microsoft.Network/virtualNetworks/jima0723b-vnet/subnets/jima0723b-master-subnet",
      "resourceGroup": "jima0723b-rg"
    },
    "type": "Microsoft.Network/loadBalancers/frontendIPConfigurations"
  }
]

From above output, privateIPAllocationMethod is static and always allocate privateIPAddress to 10.0.0.100, this might cause the 2nd cluster installation failure.

Checked the same on cluster created by using terraform, privateIPAllocationMethod is dynamic.
===============
$ az network lb show -n wxjaz723-pm99k-internal -g wxjaz723-pm99k-rg --query 'frontendIPConfigurations'
[
  {
    "etag": "W/\"e6bec037-843a-47ba-a725-3f322564be58\"",
    "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/wxjaz723-pm99k-rg/providers/Microsoft.Network/loadBalancers/wxjaz723-pm99k-internal/frontendIPConfigurations/internal-lb-ip-v4",
    "loadBalancingRules": [
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/wxjaz723-pm99k-rg/providers/Microsoft.Network/loadBalancers/wxjaz723-pm99k-internal/loadBalancingRules/api-internal-v4",
        "resourceGroup": "wxjaz723-pm99k-rg"
      },
      {
        "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/wxjaz723-pm99k-rg/providers/Microsoft.Network/loadBalancers/wxjaz723-pm99k-internal/loadBalancingRules/sint-v4",
        "resourceGroup": "wxjaz723-pm99k-rg"
      }
    ],
    "name": "internal-lb-ip-v4",
    "privateIPAddress": "10.0.0.4",
    "privateIPAddressVersion": "IPv4",
    "privateIPAllocationMethod": "Dynamic",
    "provisioningState": "Succeeded",
    "resourceGroup": "wxjaz723-pm99k-rg",
    "subnet": {
      "id": "/subscriptions/53b8f551-f0fc-4bea-8cba-6d1fefd54c8a/resourceGroups/wxjaz723-rg/providers/Microsoft.Network/virtualNetworks/wxjaz723-vnet/subnets/wxjaz723-master-subnet",
      "resourceGroup": "wxjaz723-rg"
    },
    "type": "Microsoft.Network/loadBalancers/frontendIPConfigurations"
  },
...
]

  4.17 nightly build

  Always

    1. Create shared vnet / master subnet / worker subnet
    2. Create 1st cluster in shared vnet
    3. Create 2nd cluster in shared vnet
    

    2nd cluster installation failed

    Both clusters are installed successfully.