I know that I can not probably upgrade the 2nd PC to Windows 10 but since I purchased Windows 8.1 Full version RETAIL. I want to know if the 1st upgrade will become non licensed. Thanks.
No as the window 10 upgrade is linked to your Window 8.1 licence. You not supposed to reuse the Win8.1 key in another pc unless you remove the pc that has the window 10 upgrade.