Even if you would have to always identify yourself with your real life ID, socialnumber, etc. befor loggin in a sitter could just use that too or, like duck said, tools between the OGs PC and himself, like superlong HDMI- and USB-Wires or by internet.
On german servers multiple players in one acc is normal and legal for years. And still most players, no matter how long they have played, start solo and merge every time with an other tribe mate in each world, because situations are always different and especially when spawnin togethers is deactivated your chance to end up at the same area like your friends is minimal, so you have to find a new tribe with formally unknown players.
In think the focus should lay on how to distinguish between fair merges and pushin by merging with bot accounts, what is the state the german community is in for a few moths/years now.
TW is a 24/7 game, if you want to stay single in your acc its logical and fair that you wouldnt catch up with accs, wich are in fact 24/7 online. You even get night bonus 1/3 a day to have time to sleep! Finding partners to merge with isnt harder than finding a tribe and to get around tribe member limit you want to merge as much as rational possible anyway.
btw the also german based arch rival of TW, TR*V**N, has a coplay mechanism build in.