My HP100 doubles, full, weigh in at just over 90lbs according to my shipping scale. That's with a Sea Elite manifold, bands and of course the bolt kit.
The 72s weigh about 30lbs each empty (bare, no valve). Two of them are 60lbs, bands and manifold are another 5 or so, which is 65. You have roughly 10lbs of gas in the doubled 72s @ 13cf per lb, assuming Nitrox (that figure is 15 in the double HP100s)
That should bring you to 75lbs with them full and ready to dive.
These numbers "check" - the HP100s have 15lbs of gas in them, which makes their "empty" weight 75lbs. Each tank claims to be 32lbs each empty, but I suspect that's not quite right. 9lbs for bands and manifold? I don't think so; I've got a set of bands right here and my postal scale shows them as almost exactly 3.5lbs with the bolt kit.
I KNOW the full weight of the Double HP100s is right, because I've measured it..... my guess is that the PST HP100s are in fact ~35lbs empty, which gives you 75lbs empty w/bands and manifold + 15lbs for gas for the set.
15lbs is a lot, especially when the double 72s should require about 4lbs LESS lead than the double HP100s (assuming the buoyancy numbers I've seen on the 72s are right.) That's a nearly 20lb difference in the total mass of the kit you're hauling around on land - not insignificant at all.