Wirralacars, you might want to give this a read, as well as what abcd567 wrote below…
If you have a working setup already, you may want to save the configuration you have or make an image of the SD card JUST IN CASE.
I THINK from your title you’re wanting to add a second receiver to your system, and I believe this link will be of some help: Two Independent 1090 Mhz ADS-B Receivers on One Pi (No UAT 978) - #4 by kenf3
Good luck
Gary K4GHL