We did that for several years; it worked well for us (though my elite status was with AA at the time). My partner actually had a decent upgrade rate on DL as an MVP, albeit mostly on short flights within the northeast and upper Midwest at off peak travel times.

Yes, my understanding is that upgrades clear in the following order on DL:

Elites on full Y fares
Diamond
Platinum
Gold
Silver
AS MVP/MVP Gold (not sure if MVP Golds are prioritised over MVPs)
Companions of DL elites

(I don't know where AS MVPs on full Y fares fit in; rarely relevant anyway.)

So your girlfriend will get ever so slightly higher priority than if she were just your companion. Either way, she's only getting the upgrade if there are few Delta elites on the flight.

The more important benefit to me would be that she gets some benefits (free checked bag, preferred seats including exit row but not domestic economy comfort, zone 1 boarding to ensure overhead bin space) when travelling herself, and you both get those benefits on AA when traveling together.

Alaska Airlines has excellent, friendly phone and Mileage Plan customer service in my experience. Using their awards seems to have gotten considerably easier yesterday, too: http://www.flyertalk.com/forum/alask...ive-aa-dl.html