Lynne is correct about the IP's. The lower IP side will not open until the high IP side reaches the lower IP. It won't transfill like you think.
You have two tanks, both with their own first stage. From the first stage of each tank a LP whip runs to the gas block. From the gas block you run your reg and DS/BC whips. The LP air at the block would have to move through a higher pressure gradient to make its way from one tank to the other. Which it won't.
When you transfill, you use HP-HP pathways, not HP-LP-HP pathways.
With one SPG on the higher IP tank you will get an accurate read of that tank till it nearly empty, then you dive the second tank blind.
With one SPG on the lower IP tank your gauge will read full until the higher IP tank is drained and then it will work for the second half of the dive.
I tried this for a project linking multiple tanks and worked through the same thinking you are having with a group. Here I have two tanks connected at one first stage but I also tried it with a gas block like the Z system as well. It was fun but, like the Z system, it violated the KISS principle too much for my liking.
Just food for thought: You could use one SPG if you connected two tanks using a transfill whip and DIN fittings: from a single post on tank A to post one of an H valve on tank B. Your traditional reg set goes on post two of the H valve.
I wouldn't recommend it but it would work.