I had a quick look at RXM-418 datasheet. They have given good information on selection of antennas and also some hints on antenna sharing as well.

Antennas are shared using special isolaters which provide high isolation and are low loss.

I am not a communications expert but if there is not a total mismatch, the antenna should work for you. No doubt you may not be able to achieve a longer communication distance but at least for demo and test purposes, antennas should work.

Regarding antenna sharing, never connect the TX and RX antenna points together. You will ruin your sensitive receiver with full power of your transmitter.