How can I store a paint roller for a second coat?
Wrap the roller in aluminium foil. Or in cling film. I've stored rollers this way for 2 weeks, though you do risk it getting a bit dry and crusty at the edges for that sort of longer period. For 2 hours you'll be fine.

The point of wrapping it is to stop air getting to (or getting away from) the paint that's on the roller. Solvent paint (i.e. most paint) dries by evaporating the solvent into the air - if there's no air it can't easily evaporate.

(If you're in the US, feel free to use aluminum foil or saran wrap instead...)