Just to make sure I cover the bases:
1. If you are going to be one driver in one car (with one spectator - cannot ride in car w/ you), you pay one entry.
2. If you are going to be two drivers in one car, you will have to be in different run groups -- there is no trading sessions -- and you will pay two entries. (note - if you would both be in novice, this won't work)
3. Two drivers, two cars is two entries and you can be in the same run group.
Basically, you can't switch drivers (you do odd sessions, she does even). The closest thing to that is to have one person do one day and the other person do another (although this event is not likely to offer two one day sessions, I would think that the registrar might be willing to split your weekend days - as the point of only selling two day deals is to keep from having 160 cars on Sat. and 100 on Sun.).
In any event, registration is now open at
NASA-VA. If you want to try to split your registration, email Elyse, the NASA registrar (linked at site). She's pretty cool.