OK, Small problem is that "twin 40s" could be anything from a couple of twin pot 1972 'rudes that will weigh as much as the contents of your fuel tank up to some 1990s built 3 pot Autolube Yam which wil lbe substantially heavier.
I'll keep it brief: for weight, just look around. your single 80/90 Hp replacement is about a 50/50 as to whether it's going to be heavier or lighter, then you may want to add an Aux, which inevitably tips the balance anyway. If you go 4- stroke, it s going to be heavier, although some of the really new ones are not too bad in the lard dept. Drag through the water again will depend on what's on there now. 40 is in the weird grey area between the smaller stuff & sharing it;s gearbox with something 50 Hp bigger, so you may already have 2 "fat" 'boxes dragging through the water, but then again, you may also have something quite small.
Other thing to remember is you'll need to cut a recess in the top of the transom unless you can find an XL shaft engine.
I'l lchuck this in as a curve ball. If you are thinking 4- stroke & your hull is rated for it, the Honda 50 4 stroke is about the same weight as a generic 50Hp 2- stroke...... From memory the new Yam 4-st are going the same way.... Would save a lot of transom surgery!
Do a search for posts by me on twins. I've given a few worked examples.