i have 2 cars insured with direct line. i have 2 years NCD which is aplied to both vehicles as long as i am the only driver of the vehicle. If i wanted to add another driver to either of my insurances they will put it down to 0 NCD on the relavent car.
Ive also had insurance previously with Quinn and it does cover you to drive any other car (not van or pick up etc) not owned by you whether it has insurance or not. although as said on this thread as soon as the vehicle is left somewhere it is not insured