Now I know I am dealing with a CB'r

You cannot use a splitter to connect two antenna's together, and a splitter is not a diplexer.
Even if all you are going to do is use the antenna's for receive only, you will still run into problems..
If it was a television antenna - we use a con - joiner - it basically isolates each antenna from each other, is real good for isolation to prevent multipath, but again, you throw away half of the signal in the Con - Joiner.

A splitter by nature is wasteful, a 2 way splitter should be unilateral, each port receives 50% of the signal.
BUT
When you advance to a 3 way splitter, they are not unilateral.
Unless it is a expensive splitter, you usually end up with one port having about 45% of the signal and the other two sharing the remaining 25% / 30% - between the two remaining ports.
This is the problem I deal with most often with reception issues.

The op buys a splitter, then figures that the television reduces the amount of received antenna power and so they buy a distribution amplifier and they figure that if they amplify the signal it will get better.
The problem being that you not only amplify the signal, you also amplify the noise, plus the amplifier makes some of its own noise.

So we steer them towards a mast head pre amplifier and it solves some problems but not all.
If the antenna is not aimed at the strongest part of the signal - we get little or not reception, or the reception we do get with digital signals is sometimes corrupted by radio waves arriving at the antenna at different times.

I've already explained dipole antenna's - which are a balanced antenna, or a vertical antenna.. So there isn't much of anything else to say except good bye on this one.

A Diplexer filters out the other frequency signal - and con - joins the two antenna's together on one feed line. Or makes believe to the receiver that there is two seperate antenna's, or keeps the rf out of the other side of the transceiver on a dual band transceiver.