Sending two serial signals down a single twisted pair?
First you say you have ONE twisted pair, and then you say "one of the twisted pairs".  We will assume that you really have only ONE pair and you meant to say "one of the wires of the twisted pair".

You are correct that unless you have rather low-frequency signals and a quite short (perhaps 1m) cable, you will likely experience bad to severe "crosstalk" between the two signals because they are so effectively capacitively coupled together by the twisted geometry.

There are ways of sending two different signals over a pair including time-domain and frequency-domain multiplexing. And using a Phantom Circuit.