Where's the shutoff valve and expansion tank for the leftmost heater?

Is that pex being used for the TPR line? (It *looks* like white pipe with a black crimp ring near the top, but I can't really see it)

What's supporting that vent near the 4" pipe?

I don't think you can lay that kind of vent flat like that.

If the left-most heater goes out, what keeps the middle heater from venting back down the left's pipe?