Cool bike!
It's starting to look a bit like a Honda Ruckus.
I like the way those have under seat storage and an open frame.
You could add more batteries in series for better total energy capacity. OR you could try over-volting your system by adding another battery in series. Some setups you CAN do this with, others you can't.
I know a lot of guys will add one additional battery to a ZAP Xebra because it gets them extra speed (and a little more range too) and there is enough margin in the voltage of the Alltrax controller that the controller still works. Then they just add in a 12V charger seperate for that one battery and charge the rest on the original charger.
Adding a whole 'nuther motor sounds complicated, but if you think you can, why not try it?