Welcome to the forum.
The short answer is No.
Only since X5 has it been possible to install more than one
version and then only for RH HTML. Even with later versions, if you
buy an upgrade rather than a full version, the installation will
pick this up and either or both versions will not work properly.
To do what you want you would have to buy the full version of
RH7 and install it on a separate machine.
You might like to take a look at the RH7 articles on my site.
Also see the Before You Install article at
http://www.adobe.com/devnet/robohelp
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