As long as both are aware of the other I dont see a set distance. Since most all regs are designed to freeflow instead of just lock up, changing to your backup 2nd stage, signaling your buddy by light waving swiftly or tank banging and watching your SPG closely is usually the best action. Some divers surface too rapidly and realize they could have taken their time since some freeflows take quite a while to empty a tank. Having redundent 1st stages where you can isolate the bad reg adds some comfort unless of course its a burst disc, which is sometimes overlooked and not replaced with the hydro as I think it should.