jj, please stop starting all these threads...This question has been answered already in at least one of your OTHER threads. I don't mean to be snapping at you, but just how many threads are you going to start about "I want to control motors at max speed with arduino", of varying number of motors? You are getting answers that in the long run probably will not be compatible with each other, instead of telling us (once) what you are trying to do clearly, you are hitting us in ten different directions with vague "I wanna run a motor" and "I wanna program" threads. You are going to run into problems if you are trying to put all the answers from all these into one project, but if we were looking at this from a single discussion, we could be looking for an integrated (and integratable) solution for the problem.
The answer is that you do it exactly the same way as instructed in the "how do I add more motors" thread you started, except two of the motors will be connected "backwards", that's it.. the only modification that would be a good idea would be to put a flyback diode on each motor rather than one diode for all of them (as now you will have spikes in two "directions"). Since the circuits and such are already referenced in that thread, it's probably best to have this disussion in the thread that the answer is already given in
We do expect that you are willing to do at least a LITTLE of your own thinking here... please read through the answers you have already been given before asking the same question again in a different thread. Folks are willing to assist, but please don't abuse that... running a half dozen threads on the same subject is counterproductive and is also a real turnoff for the folks trying to help.. thanks!