Sending Signals over a Coaxial Cable
Sending full duplex signals (biderectional) is not difficult but you have to have localized cancellation circuits at each end to remove the signal being locally transmitted leaving (mainly) the received signal. Telephones work in the same way - you can send and receive audio simultaneously and because of the cancellation circuits you can barely hear in your earpiece what you are transmitting thru the microphone. It's called a telephone hybrid: -



It doesn't need a transformer but one can be used to provide isolation and line impedance matching. Basically it's a wheatstone bridge where the line forms one limb and the driver (local transmission) is the "excitation". If the line impedance matches Zref, the  receiver circuit gets 50% of the line signal coming down the line to it but, more importantly, it receives zero signal from the local transmission i.e. total cancellation of the locally transmitted signal.

When driving a 50 ohm coax with two signals at one end, each has to drive thru a 100 ohm resistor in order to maintain the impedance seen by the coax as 50 ohms. This prevents reflections and the normal problems associated with reflections.

Voltage levels are whatever you wish them to be. The higher the level the less possible chance of corruptions due to external influences but the bigger the possibility is that you might interfere with another system. Coax aint perfect and it can create EMI due to the data edges or carrier frequencies.