I want to hear from others if you apply separation using GNSS and how to do it (phrase)?
Well, if you ask about RNAV procedural separation, the required distances are 80 miles...
I use "GNSS" distances very often in my airspace - stepped TMA with procedural control. Asking crew to report distance to NDB/FIX is the only way to make sure a/c will stay within controlled airspace while descending below FL100. I just say "Report distance ABC". And I never heard "unable" as a reply. If I will, the response will be: "Roger, maintain FL100 until ABC, expect descend in holding over ABC".
Returning to main thread:
There are situations when crew does not need GNSS to know distance from a beacon. Simple dead reckoning: overfly it, calcluate your GS (TAS + wind), count the time - you have distance. And that's the other reason both a/c should be outbound - those 4444 rules have been written in dark times with no GNSS at all.