When you say that the front TV has to be on to have sound, does the front TV have to be showing the DVD movie for the rear one to show a picture? Does the DVD player shut off when the front TV is off or switched off?
The reason I ask is because I renovated my system and installed two TVs, wired HDMI and put a theater sound system in with blue ray, etc. My bedroom TV would not play sound but I would see the picture IF the front TV was also on too. If I turned off the front TV everything would shut off. I found the problem was DLNA. I turned off DLNA control on the player and TVs and everything worked perfectly after that. It is a setting in your menus on those devices.
WBCCI 8653/AIR 60240
2017 Ford F-150 3.5 Ecobeast Gen 2 3.55 Platinum