The main issue for putting two piston engines in any helicopter has got to be matching the power output and speed of the two engines.
Modern computers coupled to the fuel control will probably have to be used, and that introduces a whole new set of problems.
Just think of the issues - the engines have to be running at the same RPM, and then produce equal amounts of power. Pretty complicated, and certainly not the sort of thing I'd like to do manually.
(So we're lifting off to the hover, and I've got to manually match the engines - what parameters are we going to use for the matching? RPM first - so I have to fiddle with two throttles or maybe a 4 way beep switch, then torque (perhaps), all the time I'm trying to hover....)
Are we going to have contingency power ratings on the engines in the event that one engine fails?
Loads of issues that will probably stop the project from ever getting off the ground.