It seems like you would need to be running two separate tests in parallel to achieve this. One on the android and one on the iOS device, with a lot of wait.until statements in there to wait while things are happening on the other device (e.g. User 1 calls while User 2 waits for a call, then User 2 picks up while User 1 waits for the call to be answered).
So yes, it would be possible, but I can't see the tests being very stable. If you manage it then let us know!
