The engine manufacturer can tell you the exhaust gas temperature and flow rate, the allowable backpressure limit, and the load limit for the exhaust flange. Be sure to include flexible connections so the engines can dance around a bit, and supports for the pipe.
Silencers will eat a lot of your backpressure budget; get help selecting them from a silencer manufacturer.
Mike Halloran
Pembroke Pines, FL, USA