The TBM 940 and Cessna caravan both have the PT6A turbo prop engine. The sister caravan has one single exhaust stack under the engine cowling, and almost all other small turbo props have dual exhaust on both sides of the engine cowling. Is there a reason for this and is one better than the other?
This depends on whether the engine has its exhaust outlet in the front of the engine or the back, and whether the fuselage is pressurized or has cabin air inlet vents. You'll put the exhaust beneath the fuselage to keep it from blowing fumes into the cabin air inlets unless the fuse is pressurized, in which case the cabin air is bled off the compressor and exhaust contamination can't occur.