Re: small twin outboard set up ?
I have done something similar to what you want to do with great success. If you can find a decent pair of Johnson or Evinrude 6hp motors of early 70's vintage, I think you will find the setup to be easy to use and quite reliable.
First, go through each motor thoroughly. Rebuild the carbs, ignition systems, fuel pumps and water pumps. Replace the thermostats and lube the motors as needed, including LU lube. While you are changing the lube in the LUs, check for water intrusion and reseal if needed. I am recommending all of this simply because the work is neither difficult or terribly expensive, but will insure reliable use for a long time. In my application, I did all of these things and was able to run the motors for several years, most often at WOT for hours on end, without having to do significant maintenance.
Since you will use your motors in a sailboat application, you will not need to worry about a steering system. Rather, adjust the friction lock for the steering (on the 6hp motors I described, it is a set screw in the front, middle portion of the transom clamp assembly) so that the motors don't turn easily. You may then use the boat's rudder to steer the boat.
What you will have to do is to put a dual engine control system together, or buy new one. I didn't want to spend the money for a new control unit so I took two OMC Shipmaster units and put them together as a dual unit. This is relatively simple to do and is almost identical to the factory made dual unit.
While having two motors does increase the chances of a failure in one of the motors, I think you will find that the increase in probability is neither double or anything close to double, particularly if you use very simple motors. What you will do, however, is increase your chances of having a working motor for manuevering in a harbor. In as much as wind is usually available, having an engine failure on a sailboat is no big deal because, even if you don't want to raise full sails, you can raise a reefed main or a small jib to maintain more hull speed than one small engine can provide. After this, you will still be able to drop the sails and move easily to your destination in the harbor.
I also don't believe that you need counter-rotating props, which is a good thing because you won't find them anyway. Really, the only reason to have counter-rotation is on installations with much larger motors. This is true because those motors create enough torque to cause lean in a boat, as well as torque steer. A pair of 6hp to 10hp motors just aren't going to do that.
We look forward to hearing more about your project.
is when I am in a bad situation (storms) motoring, no power is a bad thing. Sailing is out of the question. My one motor quits 
