What do you mean by merging, exactly?
By merging, I mean that you end up with one database for the entire content, but possibly with different views on this content so users see an interface they're familiar with.
Just for example -- individuals who are members of the Philforum group might spend some time there engaged in rigorous philosophical discussion, but then also spend time on the A2K forums having more general discussion (or philosophical discussion in the A2K manner rather than the Philforum manner).
Sure. And by the same token, they may also spend time on Facebook or Twitter, having yet another kind of discussion. I understand two different systems on two different websites. I understand one system on one website. What I don't understand is two systems on one website.
But on reflection, that's probably a design question that lies beyond the scope of our testing.