Personally I see no reason for needing an SPG as well as the pod. What I do see is the need to be 100% sure about which pod is connected to which gear and what it is synced to.
Marking your pod/reg set clearly with a coloured tag/ribbon/nail polish etc and having a corresponding mark on the computer or strap would be a great start (coloured bungee or something similar?). That way it is a simple visual check to say "blue reg, blue computer? Check. Red reg, red computer? Check)".
RyanT makes a good point - you miost likely have a reasonable feel for your gas consumption. When you check, is it in line with your normal? If not, is the dive you are doing out of your normal (swimming into a current or diving deeper will up the rate you use it, going shallower or drift diving will drop the rate). Use that knowledge to make an educated guess as to where you will be gas wise and check it with relation to the computer's reading. If there is a huge difference such as your consumption halving without a reason (such as a drift dive), then you can be fairly sure there is an issue somewhere.
As Rongoodman says, if one of you has gone out of air, you should ascend directly to the surface (so long as you have no outstanding deco obligation). Even if you do have a deco obligation, you should try to complete as much as possible and then ascend but never putting you close to both being OOA.