The above link covers this topic pretty good. Weight was the issue, but there were successful duel engine Montauks.
When I had a Marlin, I went through the same thoughts. Weight and cost of up keep versus the cool factor. My Marlin was just a fifteen foot boat, but a number of owner had converted them to duellies.
in one of the notes, there is a link to a 1988 Montauk 17 that has twin 30 HP Yammies. He says he gets about 30 NM/H with 3 men aboard. That's about 35MPH. So, it can be done......YMMV
