There actually doesn’t even seem to be a way to subscribe to a new community on Jerboa.
But yeah, I’m no software expert but I imagine implementing a way to automatically direct someone to a community on their own instance by say typing “!whatisthisthing@lemmy.ml”. Right now I mostly see people linking directly to instance where the community is hosted, which is not user-friendly, might overload servers, and defeats the purpose of federation. Hopefully something will be implemented soon.
At that point why not just agree within both communities to all migrate into a single community on whatever instance?