According to BPMN 2.0 specification:

If the Process is used as a global Process and there are multiple None Start Events, then when flow is transferred from the parent Process to the global Process, only one of the global Process’s Start Events will be triggered. The targetRef attribute of a Sequence Flow incoming to the Call Activity object can be extended to identify the appropriate Start Event.

it is possible to have multiple None Start Events for Global Processes (processes called inside of another process using Reusable Subprocess in our product). But compilation process with more than one None Event is failed.

For more details see: BPMN 2.0 Specification page 239.