In a twin screw marine vessel that has two engines arranged in tandem along the vessel's centerline, a drive system allows both engines to drive the propellers or, alternatively, either engine to drive both propellers independently (e.g. one in forward, the other in reverse) while the other engine is disengaged and shut down. The output of each engine engages and disengages with one of two chain drives with the use of a clutch. A third clutch is provided between the two chain drives for selectively engaging and disengaging the chain drives with one another. An under/overdrive unit on the output of each chain drive is linked to a respective port or starboard V-drive via a universal drive shaft. Each V-drive has an internal transmission for selective clockwise or counter clockwise rotation of a correspondingly driven one of the port and starboard propellers. The under/overdrive units allow for a change in gear ratio between single engine and twin engine drive for maximum efficiency.