If you have a permanent license to Photoshop (i.e. not a subscription) then your license is for two computers but on the same platform: Mac.
You can ask for a platform swap, which entails a letter of destruction of your Mac version, then you can get the Windows version license.
However, if you decide to sign on to the Photography subscription, then you can install one copy on a Mac and the other one on a Windows box for $9.99 per month forever. When you stop the monthly payments, the software ceases to work.
Don't see why not. Are you talking PC to another PC ?
On the system with CS6 installed, go to Help > Deactivate and that frees up 1 use of the Program allowing you to install and run it on another computer.