Turn off radiator with one or two valves?

Yeah, clockwise.
Afaik it shouldn't do any harm, they're just set up to allow them to be plumbed in either way. If it's stiff you wouldn't want to risk it sticking closed though.

Edit: ^^ Like he said, no real reason to go near it.

Don't close the left side as it's more than likely set to a certain point for balancing the system.

Barr wrote: »

What would happen if someone closed the left and the right was left open.

Just curious how would it affect the heating system?

Edit: I misread your post.

That would result in just switching off that single rad.

If the two valves were left fully open:
It could rob all the heat from the system, the hot water would just loop back to the boiler. Same thing could happen if someone opened the valve fully at the bottom of the hot water cylinder. This would result in no heat at any rads and very hot water at the bathroom.