hmmm, interesting question; that's a tough one... I don't think you would. Typically you would have a pony bottle (small air tank) as a back up if the main tank failed, thus you can turn on the air for the pony bottle which can typically be filled to 2000 PSI which should be enough for you to make a safe and proper ascent. the pony bottle and the main air supply go through a router block so that if the main air fails you can simply turn on the air line for the pony bottle from the router block (keep in mind you need to make sure you remember to have your pony bottle 'open' for the dive or it kind of defeats the purpose).
In many cases i would assume the reason for a FFM is to help protect the diver from certain elements in the water in which case removing the mask underwater would be unwise, or in some cases maybe the FFM's are equipped with comms to the surface or other divers. But, i can't imagine a situation where the mask would be removed to share air, thus the purpose of a backup mini tank that can supply air to the FFM if the main air supply runs out or stops working.