To be clear, I can see that it's possible to connect to multiple vehicles using different connections. So for example two vehicles on different IP addresses will work. But two vehicles whose data is arriving on the same connection but where each has a different sysid/component-id... that doesn't seem to be supported now
although multiple vehicles are rare they are becoming more popular especially with people using antenna trackers. I suspect if a tracker is used, dronekit will get confused.