[65Bowrider]

I normally don't post in this section as I'm not a geek on this stuff, but ...
#26 did win her class (we don't have to mention it was the only one in her class)
More impressive was the twin 55's performed very well on the 21 in offshore conditions.
A single Etec 115hp pushes our 19' bowrider 38mph with normal load.
We plan twin 115hp for our 21, #26. Advantages, like you said, a back up engine ... also mounting apart on transom will give less draft and better stability. Not yet proven, but like Bushwacker stated, a different prop and should plane the 21' on one engine.
We are pleased with our 3 yr old Etec.