Joseph DeGarmo :
Recently I heard that registering Windows 10 preview with your Microsoft account will entitle you to a free Windows 10 license. Will I be able to get a free Windows 10 standalone license and keep my Windows 8.1 license in a dual-boot configuration if I install the preview in dual-boot with Windows 8.1? Or should I install Windows 10 preview on a separate physical drive and get the free Windows 10 license on that while keeping Windows 8.1 on its original drive?
Info as of yesterday (June 19) seems to suggest that if you are currently running the Win 10 Tech Preview, and are logged in with your MS account....you will get a free, full, license for Win 10 when it releases.
http://blogs.windows.com/bloggingwindows/2015/06/19/upcoming-changes-to-windows-10-insider-preview-builds/
If this plays out as this blog post seems to suggest, then yes....you will be able to dual boot no problem.
As said...this is brand new info, 24 hours old. We shall see how it actually plays out.