Hey Steve, great inquiry. Have you figured anything more out in the last week with this? I’d be interested to see you show a specific example of this.
I feel as though we can add double stops to anything we want, with any adjacent string, permitting it makes sense for us in terms of notes sounding good together, and that the technical aspect of our finger position feels good, and it worthwhile for us. There’s times I’ll practise adding various double stops into a song, and sometimes the notes work well for this, and also times where some notes don’t work well to add double stop to. Sometimes it works to double stop through a whole tune, sometimes with only some of the notes. It can be a really valuable practise to work with double stops in our warm up exercises and scales. Here are some great resources on double stops with scales: https://fiddlehed.com/?s=double+stop+scale