Try for one exhaust system for both engines. Back-pressure must be minimal if engines are not synchronized.

A single tuned exhaust serving both engines, if their timing can maintain perfect synchronization. This should take the horsepower of each engine from 25 to approximately 28-30. Another consideration is to remanufacture the tail boom with ceramic cloth and combine the tail boom and the tuned exhaust / muffler.

Consider having the exhaust exit out the rear of the tail boom. This should result in less parasite drag and a reduced noise level.