Do you need to reverse the drive motors? Is you must reverse why not parallel 1 output to do the reverse i.e. drive both motors in reverse from a single output thus freeing up a potential input/output.
You should be able to get 1 output left motor fwd, 1 out put right fwd, 1 output reverse.
The motors draw a starting current of about 2 Amps and a run current of about 0.2 amps. I used two Toshiba 8080K Hbridge chips. I thought about tying an input together to save a channel, but in the end I wanted to be able to go forward, backward, turn right and turn left.
The hbridge chips I used are terrific and work very well with the picaxe. Here is a link to a brief description: http://profmason.com/?p=243