Just reuse the URL of the first one.

I also would like the ability for discussion posts. Sometimes it is relevant to multiple communities who probably do not have that much user overlap. Going back into my account to find the old post so I can copy/paste the title and content is not that onerous for one post on my computer, but it's more onerous when I use my cellphone to do this. Or if I have a lot of things I'd like to post and am trying to schedule them for the future in one big session now.

Or if I use the link strategy as described in other comments here, simply linking to my first post, would PieFed users in both communities see the comments from the other post?