Well, I'm interested in that, too.
As far as I know, you have to pick up multiple rulers of the same house (concerning the question, obviously). These rulers are by triplicity. How is that working?
For example, you asked about these two guys, both are linked to 7th House and you have a diurnal chart, so you'll look at the table of triplicities for each element.
Let's say you have Descendant in Sagittarius. That means you have a Fire triplicity, which is ruled by Jupiter (the natural ruler of Sagittarius, which is A) and Sun (the ruler by triplicity, which is B).
Here is the table of triplicities, provided by Skyscript.co.uk
Hope this helps.