I’m guessing lemmy is not capable of crossposting the same thing to two different communities in general, the same way it’s not possible via lemmy itself.
I’m not sure how it chooses which community to post to when there’s multiple. I would assume it would pick the first one? If it’s not, that, maybe it’s random?
Ye I really don’t understand how the same post in mastodon is possible to appear in two different groups, but not in lemmy. I am not familiar enough with the internals.