You are allowed two installations of CS6, but they have to be the same OS. With CC you can install on different OS's.
End User License Agreements (EULAs)
Bear in mind that CS6 is a superseded product receiving only occasional bug fixes and Camera Raw updates for new camera support. Adobe is doing no further development work on CS6. All its efforts are in the CC Cloud products.
Thank you, csuebele, for your quick answer.
Thank you, John Waller, for the relevant links.