If the freeflow is due to a mechanical problem with the seat, the freeflow will be relatively slight and an ascent all the way to the surface is a possibility.
However if the free flow is due to a freeze up in the first stage, it will be full force and a high performance reg can dump a single tank in not much more than a minute.
It's ok to try but your buddy needs to be ready to share air. the advantage of trying this and ascending (at a normal rate) as far as possible on your own freeflowing reg befroe switching to his alternate or primary reg is that you will be in shallower and hopefuly warmer water when you need to share air.
The decreased air flow requirement at shallower depths and the hopefully warmer water both give you more of a margin to avoid freezing the buddies reg. And in the worst case scenario if his reg also freezes, you are both a lot closer to the surface and are more likely to be able to ascend normally before his reg empties the tank.
A redundant air supply is preferable as it avoids the need to double up on one reg in cold water.
