My 2013 FX will do around 45 to 50 miles on the type of ride you say that you have even with a few hills. I do a pavement ride that is 43 miles round trip measured on a GPS (more accurate than the bike odo). It has a couple of steep hills on the route. I hold my speed to 35 to 45 mph. My gauge is flashing at the end but I still have a few miles left. However, in cold weather it is marginal on this ride. I wouldn't have any worries if the round trip was only 35 miles, even in cold weather. I have found the range of my 2013 FX ZF-5.7 to be nearly identical to my 2012 DS ZF-6.
The two batteries can be vastly different voltages and work together in the bike, according to Zero. However, the controller may lower peak torque, I don't know about that.
Here is measured data from the last time I did this ride:
Total miles: 43 (GPS), 45.8 (bike odo), 44 (iPod)
Moving average: 31 mph
Finish charge remaining: 1.089 kWh, indicated remaining capacity: 23%
Battery: 99 V
Air temp: 76*, Batt temp: 89* to 92*
Recharge: 4.12 kWh @ 6.5 hr
Hope this helps to answer your question.
Trikester