I'm happy to report over the last few days we have been able to successfully login to the same server at the same time within the vicinity of each other from the same home network.

Same issue here. The GF and I can't seem to play together most of the time. Sometimes it works, which did seem to be related to where we spawn. The only way we've been able to play together (twice) is if we spawn far away from each other. If we try to spawn at same time in the vicinity of each other one gets dropped from server. I did read on developer blog that the communication with server/client is inefficient when it comes to inventory items. More inventory = more messages being spammed to server and client. Maybe two joining at once pushes it too far? But then again, I'm not so sure this is related, as even joining a different server wasn't possible once one of us was successfully in a server playing - but we haven't thoroughly tested this while we've tried DOZENS of times to connect to the same server.