Well, for specific OS4 and MorphOS threads It wouldn't be a bad idea to make a separation in the forums section. For example if a program is compatible with Os3.x it would be in the general forum, but if it's only compatible with OS4 in the OS4 forum... same for MorphOS.
But at the moment I think that it's interesting for both sides to see the threads of the other, because there's not much stuff OS4 or MorphOS only...