Wow, good question but I'm pretty sure I remember looking at the Jay Flight G2's (which have more options than the regular Jay Flight) and being told that the battery comes with both trailers. We also bought another one to have two and they installed it for us. Pretty certain though that one battery is standard.
Denise, DH, grandkids, and two rescue pups
-2016 Jay Flight 29QBS Elite
-2016 Ford F-250 XLT 4x2, 6.2L EFI V8, 4.30 Elec Lock, Heavy duty Alt