Contextual Filter tied to 2 relations? I've currently got a view on a Node type (Prospects) where part of the filtering is a Contextual filter filtering a 'Sales Region' taxonomy field against a Sales Regions taxonomy field in the User. The idea is that, given a path of, e.g., /user/4/prospects, the view should only show Prospects whose (single) Sales Region term matches one of those in the Sales Regions terms for user 4. This is actually currently working quite nicely.

The problem comes in that we have just added another pair of taxonomy fields to the Prospect and User - a Company field in each. The User has one Company term, and the Prospect can have multiple. The idea is to additionally limit the Prospects shown to those who have a Company Term that matches the selected user's Company Term. The issue (which I realized when I tried to modify the view to handle this) is that I essentially need to tie the one Contextual Field (a UID) to two different Relations - one for the user's Sales Region field, and one for the Company field, but the Contextual Field UI only appears to allow tying it to one Relation. Is there actually a way to do this, or am I going to have to do something like write a custom filter to handle the additional Company filter?
A colleague and I got this figured out after I posted the question.

To clarify the original question: the situation is, you've got a view with a path with one argument (e.g. user/%/prospects, where the argument is expected to be a uid), but you need to use that argument in two different contextual filters - in my case, two different taxonomy comparisons between the selected user and the content I've viewing.  The problem is, a contextual filter only lets you reference one relation.  

The solution is to enter both contextual filters, but in the second one, in the "WHEN THE FILTER VALUE IS NOT IN THE URL" section, choose 'Provide default value', and in the type, choose (in this particular case) "User ID from URL".  Other 'from URL' types are available.  This allows you to 'share' one argument with two different contextual filters.