Re: tools
ditto, a spare prop and all the tools required to change it would be a start. A properly charged portable jump-start unit might come in handy (?) or tools that would allow you to connect a trolling motor battery to be used as an engine starting battery in a pinch (or jumper cables?). If you feed both engines from the same on-board tank, how about a gallon or two of gasoline that is separate from that one supply? (and no, not purchased at the same time). Enough rope to use as a tow line if you end up needing a tow in.
As for tools, I think it depends quite a bit on how handy you are. Consider what are the things are likely to leave you stranded, and where you boat. Which (if any) of those things could you fix on the water with minimal tools? Maybe the most important thing would be a VHF, or a cell phone and the phone number to a towing outfit, or the coast guard, or your friend back at shore?
When I'm out in an a sparsely inhabited (or deserted) inland lake, I like having redundant power (which you have), spare spark plugs, wrench, prop, drive pins (if needed), pliers, flashlight, multi-bit screwdriver, vice grips, zip ties, extra fuel line (one for each engine), rope, multi-function pocket knife and a few other small things as a minimum. Also, I always have fire starter stuff, bandaids, compass etc in a small fanny pack in case I end up having to spend the night on shore away from home. I'm usually out of cell coverage, but now often carry a GMRS radio that would allow others in my group to contact me if I went missing.
I'd be a lot more paranoid if I boated on big water or where a current could sweep me away. VHF would be added quickly.