Since you are using 2 engines, you are in a different position from
the typical experimental aero application as far as the reliability of
each individual componant, and the composite reliability of the system
as a whole. You haven't shared the particular engine configuration
you're planning (e.g. push-pull vs. left right) but the use of two
engines brings additional challenges and opportunities.

Opportunities:
Single ignition motors
Operation at high average outputs
Use of 2 stroke power plants.

Since you have 2 engines, you essentially have redundant everything,
completely decoupled. Real about my twin engine pitch at:

http://inline_twin.tripod.com/concept.html

There are very high output 2 stroke motors available around 1hp/lb.
Mainly adapted from snow mobile (Rotax) applications. Used as a
single, the realibility is below what most people desire, but as a
pair, its down right usuable.

Please share your engine configuration with the group.

Rotary- IMHO great possibilites, sports car engine- high power/weight
ratio for a conversion, fail soft operation (will not seize), 2
plugs/cylinder by default.