You can't combine the prefixes. And I don't think there is a prefix or other feature in Encore that will do this.
You could name the chapters in Premiere (e.g. "2 - The Party") or in Encore, and in Encore, use the Set Name from Link, and it might get what you are after (Button text that says "2 - The Party"). But adding the number (and name) would be manual in either case.
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