I have done the Windows 8.1 Pro upgrade to Windows 10 step before, I also did a clean Windows 10 installation after that, but have gone back to Windows 8.1 Pro. Now I'm wondering if it's possible to install Windows 10 again in a dualboot configuration and just be an Insider Preview tester and not have Windows 10 connected to my Windows 8.1 Pro license anymore (or risk that my Windows 8.1 installation gets deactivated)? So bascially I'd like to use Windows 10 for free the same way you could before the Windows 10 RTM release, is that still an option?
I have w8.1prowmc and w10rtm dual boot, no problem with w8.1 prowmc activation whatsoever, and it shouldn't have any problem for you either.