I have a vertex with split tangents.
I want to link the two thetas together but not link the two radii. Is this possible to do using link or do I have to explicitly export one of the radius values?
Linking sub parameters is not a straight task as linking ducks. You first need to export one of the theta angle, select it on the Children panel and then right click on the other theta and select Connect from the drop down menu.
It can be done via custom action when it is programmed by someone. It is a good task for anew coming coder 
-G