This exist on
CITYTALK
when you make a TRX loop test. Choose both separetly.
you have 2 results:
rx path 1= a
rx path 2= b
normally a=b, the alarm appear if a is different b, the difference is more over 4.
ULTRA SITE
in the menu, you have comparaison RSSI value.
You can see the diffrence with a number.
In all that case, the problem can be from antenna, feeder, TRX, câble, …