Ours is a similar setup, 2016 29BHDS. The rear tank is just the shower, the front tank is the bathroom sink and the Kitchen sink in the center island. Our shower also occasionally backs up, but I am glad to have an entire tank dedicated to that.
I am not sure why they don't hook them together, I am no plumbing expert but I wouldn't think it would be difficult.
2016 Jayflight 29BHDS
2015 Ram 2500 Laramie Crew Cab - 6.4 Hemi
2013 Ford F150 Supercrew EcoBoost, MaxTow (Loved and Sold)
Equal-i-zer Weight distribution system