I typically run both when offshore and one when trolling in the Chesapeake where I only need 2-1/2 knots or so. No particular reason, as Skunk said a single will push you at 6-7 easy enough, guess I just prefer to have power on both. There's not a default for the trim and tabs, I start with tabs fully retracted and engines all the way down then work from there as it's condition dependent. I'd say I'm usually no more than a bar or two of engine trim and then it's just touch and go on the tabs.
I have the old style 3.3L 250s and while they were an unexpected and unwelcome surprise cost (exhaust corrosion had gone undetected and gotten to my powerheads) they were also a huge improvement in every aspect of performance save top speed, as the gearing is the same and I used the original props that was to be expected. In general I'd describe them as just working easier to accomplish the same tasks; boat pops up on plane much quicker - especially when heavily loaded, cruises at the same RPM with better efficiency, holds speed plowing into heavy waves, just all around goodness. In retrospect the only thing I regret is not getting the digital controls but at the time it seemed like an additional cost I just didn't need and the old style was plug and play with my existing gear.