Where a battery is located on the bike can really affect the handling. Too high up and the bike will be top heavy - when pedaling hard out of the saddle it will fight being swung back and forth. To far back on a frame not stiffed in the rear, and the bike can "swim" when pedaling out of the saddle. When the battery is located in the frame, it keeps the lines clean and look is stealthy - so that your eBike will fit in more with regular bikes.
Dual batteries are boss for long tours with lots of luggage, routes with steep climbs, or if you just don't want to have to worry about recharging Riese & Muller bikes with the dual battery have special smart circuitry that shifts load on the batteries by alternating the load in 5% increments. This increases the service life or "charge cycles" - the number of times the battery can be completely drained and recharged. A 2nd battery will add about 7lbs.