Is it possible to have IP addresses belonging to different subnets on the internal interfaces connected through WAN for sync purposes when deploying multi-site cluster?
I think it's dependant on the cluster type. An Active/Passive cluster working together is recommended to be on the same subnet, although that subnet can span multiple sites if you trunk your VLANs between the sites.
An active/active cluster can be on different subnets as they are effectively working seperately from each other.
You could put the IVEs in an active/active cluster with DXs in front of them, then place the IVE targets in global server load balancing group.