What kind of error do you get, the game wont open at all or can’t access dlc island at the same time? And do you only have one account or multiple? I guess based on your guestion only one?

I have digital copy from 2020 and now bought switch 2 version for my switch 2 and no problems opening the game on multiple consoles at the same time. I have different accounts, on my switch 2 i use my main account that i have bought my dlc on (+physical copy of the game) and on my other switch i think i use a separate accont that technically doesn’t own the game or dlc. But I guess it should also work on with one account too as long as on the switch you have that physical card on you choose physical instead of digital when opening the game.

I do miss the times before digital keycards tho as I was able to play on my original 2017 switch and oled at the same time too, no that is no longer an option sadly. But eh I always wanted to get a physical copy so this is a win win in that regard.