My first impression is to leave things the way they are, but just add a better tutorial/FAQ on how to search threads using google. Have the link to the: "here's how you search ADV tutorial" in the ADV 'search this forum' pull down menu.
My rides are old Honda 500 cc RFVC and I would argue that they belong in the same forum with the 600s and 650s since they share the clutch/bottom end and also the valves/cam specs. I don't think anybody will suggest that discussion of my 83 & 84 XR5s needs to be in "old's cool"
There are practically no engine parts interchange between my bikes and the XR400s. So just be careful about where you set the split if its going to be based upon displacement. It would suck to split bikes that actually share a bunch of tech.
But then again, I've been treating the Thumpers forum primarily as a tech forum. Maybe I'm wrong about that?
I do agree that there should be a separate forum for 2 stroke MXers and Enduros. I could never figure out if the '2 smokers' category in old's cool should include my 77 IT400.