when i connected the motors to the picaxe via the darlington transistor array i got full speed on both motors but when i connected the motors to the L293D i only got half the amount of power in forwards and reverse.
I would advise against it for 2 reasons. 9v is slightly too high for most cheap 6v rated motors, and the normal 9v rectangular batteries are very underpowered and can`t deliver enough current.