I have to admit I don't quite understand your question, but as a procedural app/twr controller I'll explain my separation(I hope this is what you mean).
We use VOR STARs from entry point until Intermediate Fix, where the aircraft should start intercepting the ILS. The most used STAR doesn't have an ARC DME, but we have a couple that have.
First let's take the non ARC procedure. I know with a 0.5-1 minute error the time it takes an aircraft to reach the IF from entry point via STAR. So what we usually do is put 4-5 minutes between them and the first is authorized for STAR, the second descends min STAR FL+1000 ft and from a point approve STAR. I can also restrict the descent to a minimum altitude, providing separation from the STAR, until the first aircraft is ILS established.
Second, ARC procedure. We usually approve normal STAR for the first and second descending to separation FL on course to the VOR and the issue a different STAR from VOR to IF. This way the first has enough time to establish.
If you have(we don't), use points to separate. Let's say you have ABC point prior to IF(point that maintains separation ofc). You can approve ILS for the first one and the second approved ILS via ABC. It depends on your control zone.
You can always ask for position reports or if you have them in sight tell them(here at least it's a controller based separation) or have them separated from each other if they see each other.
Hoped I helped a little.
Cheers.