There are issues here. Dual insurance generally causes debate as to which company is liable for paying out, a 50% from each usually happens here. It causes no end of delay and debate. Both companies need to decide on the repair or write off costs which is not easy.
If a car is parked and rolls down the hill due to a badly maintained handbrake is this down to the driver or the owner? (There is a legally defined answer to this but I'm using it to highlight the complexities here.)
Some policies also specify that if a loss is covered elsewhere then it will not be covered under this policy!
Attempting to make the same claim twice is Fraud, it will always be detected and will lead to prosecution.
There is also the issue of insurable interest. You can't insure your neighbours car, set light to it and claim for it as you have no insurable interest in the car. Spouses can insure their partners car though as all assets owned in a marriage are considered joint.
You can draft up a contract to cover insurable interest and outline the conditions under which the driver is liable for arranging his own cover. Generally if the damage occurs whilst in his custody or control he is personally liable for it.
Any insurer must be informed and given a chance to see this contract.
There are options now where a driver can take out cover for another car. Temporary policies offer a good solution here but cost quite a bit long term. They are acceptable to most insurers and in most situations.