A loss of a 1st stage on a dive that requires doubles is a thumbed dive anyway, as long as your Min gas or Thirds gas planning is correct, with a failed 1st stage, or blown burst disc, there will always be enough gas for an exit, return to a shotline, or immediate ascent with an smb.
Part of the base of the safety/skills pyramid that you build all of your other skills on is that you or your buddy will donate a regulator that they were just breathing, given that their situational awareness lets them know that you are exiting or ascending with only one first stage, in the event of a second equipment failure.
Therefore the loss of the SPG is a moot point given that you and your buddy are trained and practiced well, prepared for the dive and follow the procedures that you trained on