I've run the thought experiment of carrying two with manifolded doubles, one on each post. I'm a little surprised that's never done.
My experience with analog pressure gauges in other environments is that, eventually, they all start to stick.
unnecessary on doubles and can complicate things. Even in a cave, if a failure occurs, you just exit. You either have enough to get out or you don't, and you sure as hell aren't doing anything except getting out if you have a post failure.
The complicating bit that our good friend AJ didn't feel like explaining is this, and he'll yell at me for saying it because I'm not GUE trained but I don't care
GUE/DIR setups do not put a D-ring on the right hip. This is because the canister light goes there and serves double duty of retaining the long hose. If you put an SPG on the right post, you have to put it somewhere. You can't put it on the right hip because then you have to ensure that it isn't crossed with the primary hose for long hose donation and there's no more room on your right hip because of the canister. So you bring up right shoulder, which is another potential location for it, but again, you have to ensure that it isn't crossed with the primary, and then it can become a catch hazard if you are in low bedding planes and you can't stay far enough above the line. You could put a super long hose on there and run it over to your left hip, but then you have another SPG down there and no real way of verifying which one is which if they match.
When I dive I/D's I actually run them down and clip to the shoulder D-rings on rather short hoses, but that isn't compatible with their philosophy, and since they largely set the standards for backmount doubles diving configuration, you won't see double SPG's any time soon.
I have seen SPG's stick, but the weren't being taken care of and had salt water at some point get into the bourdon tube because someone was careless with rinsing. The failures that you see all the time are those two itty bitty o-rings on the spools, and those leak all the time. Putting a second SPG on when you don't have to is just asking for trouble.