On some units (such as mine) the external connection comes inside to a dual coax wall plate with a short jumper between the two coax connectors that then goes to the TV(s). I can remove the jumper and connect that type box to the two coax connectors on the wall plate. That will send the cable box signal to the TVs via the internal wiring from the outside connection. BUT. With a single cable box both TVs will get the same channel since the box can only be tuned to one channel at a time.
Look to see if you have such a dual coax wall plate.