Well, you can run the two version on the same machine with
caution (it is obviously not recommended).
However, personally I would not recommend switching between
version and source files in the two versions. Hopefully someone
else will pop up if there is a way around this, but my guess is
that it will be fiddly and you risk getting in a right muddle as
the underlying database has been updated for Robohelp 6 along with
lots of additional features.
Once you have updated your source files to Robohelp 6 it is
best to edit manage the files in Robohelp 6. That's my take.