You may not be surprised to hear I did look into this before someone made me an offer I couldn't refuse for my old SR4 hull, and in summary I would have gone down the route, just not using twin Suz DT25s:
Yes, you'll need short shafts
Will they fit? - Depends if you have an A- frame and what engines you pick.
Weight - you will need to be very careful choosing your engines. Most 25s are around the 70-75 KG mark. Having said that, if you don't mind a bit of oldschool premix twin pot action, one that jumps to mind is the old Johnsorude 25 of the late 60s / early 70s which was a nice & light 38 KG or thereabouts. Noisy as, but light. And of course have you added the aux into the equation if you feel you need one.......
Usual stuff about the cost of two sets of remotes etc etc, but if you go pull start twin remotes with no keys etc probably won't be too much worse than a single with electrics. If you are buying new, cost wil lbe a lot more, secondhand will obviously depend what you find.
As for the drink like a fish, I doubt it. We're only talking 50Hp total here on a boat that weighs a couple of hundred kilos. Also if your 40 is as long in the tooth as you imply, I bet the difference in consumption is at best negligible. Smaller engines = smaller gearboxes = less drag.... Do a search for "twins drag".