[Brian C]

If your questions is "How?" the answer is pretty much that you can wire them in either serial or parallel.

If you wire them in parallel you will still have 12 volts but twice as much current available. This may also mean larger conductors of you plan on drawing that current quickly. If you did this in your currrent robot configuration now it would basically extend your run time to approximately double.

By wiring them in series you will get 24 volts and about the same amount of current capability you have now. Obviously this would mean a change to any components that would be sensitive to this higher voltage.