Would anyone know if it is possible to run robohelp 11 and 2015 in parallel on the same machine?
Definitely possible. I've done it for several months without any issue. The only issue you might encounter would depend on how you choose to open projects.
Generally speaking, the newest version is also the last version installed. And the last version installed typically mandates the behavior of the Windows File Association. So if you are accustomed to locating the XPJ file and just double-clicking to open it up, the undesired version of RoboHelp may open.
But as long as you open projects by first opening RoboHelp, you should be fine.
I hope Rick won't me adding that adding a shortcut to the different versions in your Send To folder helps if you work from File Explorer.
See www.grainge.org for RoboHelp and Authoring tips