Have AI or have an SPG. Having both is the worst option.
It's funny how basically everyone agrees that it's fine to have just one SPG and no AI. But, some people will say it's not okay to have AI and no SPG. Essentially, that is hypocritical.
Yes, AI can fail. But, so can an SPG. Your procedure following a failure is the same, either way.
These days, when talking about a PPS or Shearwater transmitter, I don't see any evidence that SPGs are more reliable than AI. So, if diving with only an SPG is okay, then diving with only (PPS or Shearwater) AI should be okay, too.
Personally, I have used AI since the first dive after completing my OW cert. I have used AI on every dive. I have only used an actual SPG on a relatively few number of dives (for my primary gas - I do use SPGs on deco and bailout cylinders). And despite the much less usage, I've seen/had more failures from an SPG than from AI. I have never yet (in 800-ish dives) had to miss a dive or end a dive early because of an AI failure.
Once you have AI, adding an SPG just adds something like 4 O-rings that are each a potential source of failure (and gas loss) plus a HP hose that is also a potential source of failure and gas loss. All that for no real benefit.
Adding an SPG also makes it where you can put your rig together and check your SPG to verify your cylinder is full - forgetting your computer or not verifying that your computer and AI is working. I used to have a button gauge on my single tank reg set. I took it off so that it forces me to get out my computer to verify cylinder pressure before a dive. That, in turn, ensures that I don't forget my computer (e.g. leave it in the truck when my boat heads out) and that my AI does not have a dead battery or some other failure.