On Fri, 25 Apr 2008 22:30:45 -0700 (PDT), Jay
wrote:

If a person strapped TWO of those Suzuki 2.5hp outboards (only 29 lbs.
each) to the back of the boat, one on each side, what would be the
effect? 5hp? Double the speed? Double the noise? Nothing? Just
curious...

If you want to go significantly faster you need enough power to get on
plane. That will take 10 to 20 hp for your boat depending on how
heavily loaded it is. The complexity of trying to steer two outboards
is reason enough not to do it but there are other reasons also like
increased drag, fuel consumption and maintenance.

Most dual outboard installations are done for redundancy on offshore
boats or where the boat is heavy enough that no single motor provides
enough power.