When I dive a back mount configuration with a stage,the long hose 2nd stage is clipped to a chest d-ring,and the stage reg is in my mouth. I am not a fan of donating a stage reg in a OOA situation since now my clearance between the OOA diver and myself is 40",less room for me to address the situation-it could be very difficult to unclip a stage when there will be very little slack on this hose,more likely it will be pulled very tight. My preference is to hand the OOA diver a 7' hose,quickly deployed by pulling on the 2nd stage,breaking the o-ring,and getting 7' of separation between myself and the OOA diver. Once the situation is under control,then my preference would be to hand the OOA diver a stage for greater separation,and stow my long hose. About 6 years ago when we were doing some big dives in a system,we tested some of these scenarios to find which would be the most efficacious,and had the greatest success under testing with this scenario.