You can run them independent in most cases. That thing is probably heavy enough where you would want atleast 2 running.I know! He's bringing spare parts.
Seriously, I think I would've gone with a marine LS motor. Similar or less weight, less complexity, similar power. But less 'bling'.
I'm ignorant with boats - how do multi-engine boats like this link together to put the power down?
Guessing he might not always want (or be able) to run all engines at once. How does that happen?