I assume you have one of these. https://www.gme.net.au/getxrs/ Try ringing the company and ask them. I called in to see them in Sydney five years ago to ask few questions about an emergency locating beacon and they were very helpful.
Welcome to the forum.
Adding another antenna will probably make it worse & it would require a matching network to make it work for very little gain. You would be better off investing in a good high gain antenna instead.
What @Ditch said...
Invest in a good antenna to start with, rather than trying to phase match two antennas - if you get the matching wrong (which will be most likely 99.9% of the time) you will definitely make reception worse and run the risk of blowing output transistors on transmit.
You would be relying on trying to match two antenna and lead lengths to sub-millimeter accuracy.