I really should write a macro to auto type this lot......
Drag etc at the 25/30 knots you are talking about on an SR5.4 is nothing compared to the drag of pulling a 2/300HP gearbox through the water at 45 knots. Also the 'box on a 40 will be smaller than that on a 60/ 75. As a very rough guide drag goes up as the cube of the frontal area and the square of the speed. (it's a good chunk of the reason you drink fuel flat out but a modest throttling back gives a disproportionate fuel economy increase). Net result is this "-20%" or whatever the rule of thumb keeps getting quoted for the larger end is not really applicable down here.
As Pol says, you could get away with no batteries. Pretty much all 40s will run standalone, and have a readily accessible start string at the front.
As for twice the juice, bottom line is that if you are running flat out there isn't going to be a lot of difference betwween 80 total horses and a single 75. if however you do a lot of stuff like dinghy rescue at regattas, you can pottle about on a single 40, and fire up the other one as you see a sail turning into a centreboard!
As for the fuel contamination, the most likely source is seawater. You don't have the problem of underdeck tanks on a SR, and lets face it, apart from a couple of years ago, how many "poison fuel" eposodes have there been? Twin Hulks will mean if one ingests water you stand half a chance of getting back.
....and the other one that bizarrely hasn't been mentioned yet is weight. If you take any range of outboards and compare twin n Hp with a single 2n Hp, the total weight of the rig will alternate as you wander up the total horsepower. It's down to every manufacturer usung the same hardware and de- tuning for a smaller Hp to get two or three engines out of one set of components. (Keeps the manufacturing costs down) Net result is that at the Sub 150 HP as a genral rule if you pick a randon total HP, it's 50/50 as to whether the twins or the single are lighter. Then add 10-15kg of Aux and inevitaly the twins win on weight.
So, my advice would be to look at what you can find, and do a deep dive comparison. Ask to measure the gearbox / leg size at the bottom, you may be surprised just how much smaller the twins' boxes could be. As I guess you are not buying new it's even more important to compare, as you may find the twin "Engines'r'us" combo is the lardy option at that horsepower, whereas the single "fuelburner inc" may also be the heavy one so you are doomed either way. To put it in perspective, my Merc clamshell (60Hp) weighs 7Kg more than my old Suz DT25. It also has an identical diameter gearbox, unlikle the old Yam 55 that the merc replaced - that I got leg spares for off a 90!
There is no generic answer......