There are two ways to do this:-
Either: use the previous 600mA circuit but replace the L293D with a Texas Instruments SN754410 which is a direct pin for pin replacement. This is the preferred solution as the chip costs 98 cents and you dont need the additional diodes.
Alternatively:- do the following.
This circuit is similar to the 600mA version except that:-
1. It uses the 'L293' rather than the 'L293D' See http://focus.ti.com/lit/ds/symlink/l293.pdf
2. This chip allows a higher current of 1 amp per motor but requires us to add the diodes at the output stage. These should be fast, ie Schottky, diodes that can cope with the 1 amp current and a reverse voltage greater than, or equal to, the voltage of your motors. The motor voltage is still limited to 36v.
This requires one tri-state switch per motor;
one L293 for every two motors. The L293 costs about $5.81
You will also need 4 diodes per motor. The cost of these will vary depending on the voltage of the motors, and the current, but lets say about 50 cents each - ie $2 per motor.
So about $12 in total - for driving two motors