Regarding the batteries, I have to say i'm not totally sure, but i think you'll get 24 volts if you connect the two batteries. We have a Volvo loader (on the farm) that runs 24 volts, and it has two 12 volt batteries - but again, I'm not sure if you can't get around that in some way maybe.
Good job on the bike! The color scheme really isn't my taste, but everything else I like!