Hi there
I currently have the following all installed and peacefully
coexisting side by side on the same PC.
RoboHelp X5
RoboHelp 6
RoboHelp 7
RoboHelp 8
So I'm going to go so far as to blatantly claim that version
6 and 7 together will work. What you can't do is to expect to use 7
to edit projects and still use 6 to edit the same projects.
There is another issue that might be at play too. If 7 was
purchased as an upgrade for 6, the licensing probably prevents you
from leaving 6 installed and also installing 7. You would have to
have purchased full versions of each to do so.
Cheers... Rick