It seems almost all commenters here are agreeing with the premise that ‘posters [have] to crosspost to both’.
I don’t think this is true. It leads to people subscribed to both having two identical posts with different responses in their feed, which is annoying. Just post to the one that you’re ‘closest’ to, or pick one at random.
Part of beauty of federation is that you can see all the content from multiple places. Cross-posting is not required!