Hi Tarun,
Thanks for asking.  Yes, on 24 Dec I managed to get Windows 10 on, 'over the top of' the second Windows 7 for which I had purchased a new key. It took a couple of all-nighters and some expert help. It turned out the problem with both drivers and installation was an incompatibility between the W10 installer and the RAID controller on my Dell Precision.  I now have a Digital Licence for W10 - and so far, my main W7 is still  showing 'activated'. I won't feel completely confident until W7 passes its deadline on 14 Jan but - so far so good. There is an absolute torrent of updates arriving for W7 at the moment and I hear reports of things breaking. 

Edited December 30, 20195 yr by orcadian

You can use the Windows 7 licence key on either one or the other without breaking the law. You can reinstall Windows 7 using the licence key if you remove Windows 10. With a single licence key, you can only run one or the other.

Although I don't believe there is a strict enforcement mechanism to prevent you from operating both, it would be a licence infringement.