Multiple version of PBP on same computer

I read a while back in a thread about the following:
Multiple versions of PBP on the same computer.
I.E. V2.42,2.50X, 2.6X etcetera.
The reason is that some versions do not support newer versions of MPASM?
I think that Darrel an others came up with a workable solution.
Can not find it now when I need it!
Can anyone remember where this is?

Life IS a B*)#^ when ones memory starts to go.........

Terry