Does both engines have to be of equal HP? Or can I have a 70 HP and a 65HP on the back? Wouldn't that mess with my controlling of the boat?

I'm no expert but I'm gonna say yeah, if you have 5 more hp from one engine you're gonna push to the opposite side of the more powerful engine because it has more thrust. 

That said, why would you even want to do that?  The only multi-screw setups I've ever seen are on larger ocean going boats and the engines are usually 150+ hp. 

The 5 hp difference is only a minor problem.  One of the motors needs a counter rotating prop or the steering torque will be tremendous.   One lower unit is left hand rotation, the other right hand rotation.

I'm no expert but I'm gonna say yeah, if you have 5 more hp from one engine you're gonna push to the opposite side of the more powerful engine because it has more thrust.

That said, why would you even want to do that? The only multi-screw setups I've ever seen are on larger ocean going boats and the engines are usually 150+ hp.

 

I would want to do it, well......... for more horsepower. The counter rotating prop makes alot of sense though. How does that work? Is it possible to reverse the direction of a lower unit? I've seen plenty of older Johnsons dual mounted. I thought it would be a good idea, but its seeming to be too much of a hassle. Just curious. Thanks for your input fellas.

I thought it would be a good idea, but its seeming to be too much of a hassle.

That was kind of what I was driving at when I asked why would you want to do it.  Not only is there the hassle of getting it set up but now you have two motors to break down instead of one. 

I mentioned that I've only seen twin screw setups on ocean going boats and I think there are a couple of reasons they are used on them.  First, having two or more engines when you're fishing way offshore is actually a good thing.  Getting stuck without power when you can't see land is a whole lot different than getting stuck on a lake where you likely have cell phone coverage and can call someone to help you out. 

Secondly, the size motors they run in tandem (or more) on ocean going boats typically add up to more horsepower than you could get in a single engine.  That's not the case in your situation.

Ah, I never thought of that one. Two 200 HP's equal 400HP which is too much HP for one engine. I'm now thinking of just switching from a fiberglass tri-hull, which is very heavy, to an aluminum deep-v. That should gain me some speed, and save me some gas. Thanks for pointing me in the right direction.