I have two different pc and I made backup on one external hard drive two different partitions but now I do not know whitch image backup belongs to witch pc If by mistake I use another image backup to restore what happens?
Most likely your computer will boot and run the Windows install from the other computer and everything will work fine, except that Windows may not be activated. What backup software are you using? It should be possible to mount your backup, so you can access it from Windows explorer and browse its files, which should help you tell which backup is which.