It is not clear as to whether there is a requirement for SM 2.x and 3.x to be deployed on the same cluster, though important to note in this bug that it is currently not possible to do. This is because, at a minimum selector names for both operator versions are the same "istio-operator"

Error:
install strategy failed: Deployment.apps "istio-operator" is invalid:
spec.selector: Invalid value:
v1.LabelSelector{MatchLabels:map[string]string

{"app.kubernetes.io/created-by":"sailoperator", "app.kubernetes.io/part-of":"sailoperator", "control-plane":"istio-operator"}

,
MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is immutable