If you take it offline you can play on both at the same time, for games that work offline. Only useful if you have wait for it games like MMOs waiting for a group etc.

Thank you all

you can. steam will send you a authentication code to your email that's associated with your account to verify its you when you sign onto a new device.

Note that the same Steam account can be active on several devices, but only one of them can start a game.