Your point form sentence template could be simply: "Parentage: [Parent]."
Much thanks for the suggestion. It has been very thought provoking.
I had settled on a sentence template of "Parents: [Desc].", but your suggestion will also work. Using the Description field, I would just set the value of the Description field to "John Doe and Jane Smith" Using witness roles, I would just set John Doe and Jane Smith with the role of Parent and as you said the the sentence template would be "Parent: [Parent]." But this is where it gets to be thought provoking.
First of all, I don't work very much with roles in RM and I had forgotten how a variable such as [Parent] works when the variable is a role and when there are multiple people with the same role. In the case we are talking about, the single variable [Parent] expands into "John Doe and Jane Smith". And if (for example), several individuals were assigned the role of bridesmaid at a wedding, then the variable [Bridesmaid] in the sentence for the wedding fact would expand into a list of all the bridesmaids, complete with proper punctuation, e.g., "Sarah Williams, Elizabeth Johnson, Tiffany Anderson, and Brenda Fox". The is all very well done on the part of RM.
Second of all, exactly what does it mean in RM to set a role? Well, it means to share a fact. Or at least I don't think you can set a role without sharing a fact. And at first blush, it would seem that sharing a fact means that there is a sharer's version of the shared fact and the sharee's version of the shared fact. RM supports the sharer's version of the sentence to be different from the sharee's version of the fact, and RM supports the sharer's version of the fact note to be different from the sharee's version of the fact note. In the case of the Parents fact I was playing with, I didn't really want a sharee's version of the fact to show up in the timeline for the parents, so I set the sharee's version of the sentence template to null. But the sentence showed up in the sharee's timeline anyway as a dangling citation superscript not really connected to anything. Then I remembered that there is a solution. Namely,when you share a fact, instead of giving the name of a person in the RM database, you can just type a name without referencing anybody in the database. The name is just text, and the act of "sharing a fact" doesn't really share a fact. It just sets a role, which is all I wanted to do anyway. So it works just fine to set a Parents role without really sharing a fact and to use a sentence template such as "Parents: [Parent]." It's just that it's a little odd to use the "share a fact" dialog to set a role instead of sharing a fact, but so be it.
Third of all, we now have two solutions. Put the parent data into the Description field and let the sentence template be "Parents: [Desc]." or put the parent data into a role and let the sentence template be "Parents: [Parent]." So which is better? I guess it depends, but my sense is that the Description technique is more likely to survive export into third party software, so that's the one I'm going to go with.
By the way and a little off subject, sentence template customization for a sharee's version of a shared fact is not exported in RM's GEDCOM, even when the "Extra details (RM specific)" option is chosen for the GEDCOM export. Don't ask me how I know this. I haven't checked to see what RM's GEDCOM looks like for a role when there is just a bare role without also having a sharee's fact.