I had a triple boot system, Win 7/Win 8.1/Win 10 Preview (Insider) V10240 - all on separate drives.
I tried to upgrade the 8.1 installation to Win 10 and it failed on several attempts with different errors. I physically disconnected the Win 10 Preview hard drive from the system. Did a cold boot and was able to upgrade my 8.1 to Win 10 and now have a dual boot Win 7/Win 10.

I had a previous post/discussion on whether I could have a system with two Win 10's installed. I wanted my "production" system to be upgraded but I wanted to stay with the Insider program with the Insider Win 10 install. There was never a resolution on that thread other than try it and see. The answer, apparently, is No you can't have two Win 10 installations on the same PC, at least not as a triple boot.