You just put an @ and then the url of the other instance, then sub to it like you did the first diablo4 magazine
/m/diablo4
/m/diablo4@lemmy.ml
/m/diablo4@lemmy.world
/m/diablo4@sh.itjust.works
/m/diablo4@xxxxx.xxx
I believe the developers of this entire thing is thinking of ways to easily get it merged in a way, but still allow you to opt out of specific ones, because the moderating on one instance may be complete garbage, and you won't want to see it.