Style sets are nothing else the dotx Template files. So to have different style sets showing up in the Word user interface, you must create different templates. In your case, you'll have "Long agreement.dotx" and "Short agreement.dotx" in your Word QuickStyles folder, right?
To have different multilevel list bound to different style sets, you must open the appropriate template and define in a first step the multi-level list format in this template. You take care to bind the different level to the built-in heading styles 1-9. In a second step you format the heading styles in a way appropriate to the template, e.g. for the Short agreement you use smaller fonts and smaller paragraph spacing.
You'll end up with two distinct templates, which represent two style sets. Each template has heading styles bound to a list definition. As of my own tests, if you apply the style set to a document, the styles are copied together with their list definition - even when this list definition is not visible in the List Gallery.
From a user's point of view, this seems to be an approach which is easier to understand. Users shouldn't care about the combination of List Formats and Styles, but should only apply the style set.
As of my tests, there's no efficient way to ship List Formats as such to computers or users in a corporate environment. The only way is to bind them to styles, and ship them with templates (or create them through code, as the "Numbering Tools" on the market do). In my eyes, this is also the better way to have users apply correctly multi-level lists, because multi-level lists are used in 99% of documents with the idea of structuring the content. So a lot of misunderstanding and direct formatting happens, if List Formats show up in the gallery without relation to heading styles.