Unfortunately I do not know of a way of accomplishing this construct in a proper fashion. You can kind of fake it by making the sub line a level two list item, delete the bullet with backspace, and then apply the ruler settings from a level one to that level two item—but it’s important to note that this item does not cease being a level two item even if it looks like a level one secondary paragraph. It is thus something you’d only want to do after that initial nested list is complete. Changing it later on will cause the number to re-appear on the sub line and you’ll need to clean it out again. Since it is considered a level two outline item though, it will not conflict with the top-level numbering—so it works well with edits at that level.
However like I say—that’s not really the proper way to do this and it may very well result in a mess when going to a word processor later on, so I’d test it thoroughly with your full production chain before adopting it.
Would it work to hit enter again (until you reach top “level” and the text system recognises you are no longer in a list) and then create a formatting preset to match the desired indent level (you could call it List 2 discussion)?
I have successfully used a similar work-around in Pages.
I think that would break numbering on the top level, because now there is a paragraph in between the two lists—no matter if it is formatted to look as though it were a part of each list to the top and bottom of it. If I do that anyway, in a 3-point list with a sub-list on #2, and hit enter until I’m out of list mode after the #2 sub-list, then item #3 below gets turned into a “1” as it is now a new list below the paragraph you’ve just inserted.
I’d forgotten that Scriv doesn’t have the option of continuing list numbers, but that doesn’t matter since you can start a list at any number, so you could just tell Scriv to “start” at 3 with the third item. Even though Scrivener thinks it is a new list, it will look like a continuation of the original list. The downside is that it won’t all renumber nicely if you change your list, but it will achieve the desired visual style.
Just tested in Scriv and it worked.
Some notes about two point three.