#missing-knowledge
3 items (3 fixes)
A second editor instance launched with -joinlocal is not a real Steam lobby member, yet Networking.GetData still resolves the host's Networking.SetData values -- they arrive over the ServerDataMsg the host sends on connect, not via lobby membership. The message can lag scene load by a few frames, so read GetData in a short poll loop.
A -joinlocal second instance connects to whichever process holds the loopback port 127.0.0.1:55333, so the precondition to check is socket ownership, not which editor is in play mode.
sbox.exe -joinlocal +instanceid 1 gives you a real second peer against an editor host with no publish, no second Steam account, and no lobby discovery. It has no host selector: it joins whichever local editor is in PLAY MODE, so no other editor on the machine may be playing.