I think it'll depend on which app.
I know, for example, that you can have multiple versions of iMovie installed at once without issue--I've done this several times in the past. I assume Garage Band would be the same.
The apps with a library, pretty sure that won't work--iTunes almost certainly not, but maybe if you pointed two different versions of iPhoto at different libraries it might work.
With iDVD it comes down to where all the extras are stored--if it's all in-package, then two should be fine. If it's looking for resources in an Application Support folder, probably not.