Control of a vehicle powertrain with multiple prime movers

A vehicle powertrain having multiple prime movers including a primary prime mover coupled inline to a transmission and a secondary prime mover selectively coupled to the transmission by a substantially concentric coupling. A method of powertrain control is also disclosed, wherein powertrain torque demand and torque available from the primary prime mover and the transmission within its present gear ratio are determined and compared. Output of the primary prime mover is increased if the powertrain torque demand does not exceed the torque available. Otherwise, the transmission is downshifted to a lower gear ratio. Torque demand is compared to torque available from the primary prime mover and the transmission within its now downshifted gear ratio. Output of the primary prime mover is increased if torque demand does not exceed torque available. Otherwise the secondary prime mover is started, its output increased, and the transmission is shifted to a higher gear ratio.