There is no one answer since it will depend on the type of brushed motors you have. Some brushed motors can go both forward and reverse... but some are only meant to spin in one direction. If these motors can be reversed, just reverse the wires to one of the motors. If they aren't made to operate in both directions though, you could ruin one motor or it may just not operate as efficiently and will run slower.
The following is a general way to find out which it may be but it's a crude method... so your results may vary...
If you can see the brushes and commutator ... look at the brushes and make certain they are perfectly perpendicular ( 90° ) and centered completely on the brushes.... If so then hook up a single battery to one and listen, do the same for the second motor to hear if it sounds like it's spinning the same speed. If you have a tachometer that you can hook up, it will tell you more accurately the speeds than just listening to it.