Usually for fully comprehensive insurance, you are allowed to drive somebody's else car with the owner's permission. However you need to check whether this is included on the policy by looking at the policy documents that the insurance company sent out to you.
If you can't locate the information, it is in your best interest to call up the insurer to confirm everything before you go ahead and let the named driver use your other car.
It is also worth noting that named drivers on a policy have the same facility as the policyholder, so if you have comprehensive insurance cover as the main driver, then any named drivers will also have fully comp insurance and if you are allowed to drive other cars, then named drivers will also be able to do so.
Mind you that when driving other cars, you will be covered under third party only insurance and not comprehensive, so any damage that is caused to your car whilst the named driver is using it will not be covered.