Hi, Paul,
If this is difficult to implement, don't worry about it, but depending on how things are set up, there's a chance it would be fairly simple to do.
Some topics belong in several fora; the example I have in mind is the "whole
trees" topic, which belongs in both woodland care and alternative building.
One of the problems I foresee in cross-posting is that any post in the topic could potentially count as activity in each of the fora it was posted to, giving cross-posted topics unfair attention and perhaps confusing or frustrating readers with the redundancy. It might be worthwhile to count a new post to a topic only as activity in the forum that the user was in prior to posting.
Again, I just wanted to float the idea...there might be lots of reasons not to.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.