That depends on the airport! How are you both going to drive out of the airport if it's one of those where you pick up the car and then the only place they check your drivers license is at the exit? By then, don't each of you have to be in your own cars? Is the second driver allowed to take the car out of the lot (at any airport where a Preferred car is checked against the driver only at exit), or only allowed to drive it later (after leaving the lot)???

I don't see how that's cheaper than your wife signing up for Preferred herself and then using the same AWD you do. You should then both get the same rate. (Of course, on her first Preferred rental ever, I believe she'll have to visit the desk and show the linked card.)

Now, if you know for sure this is a location without a Preferred board (ie, a small location where you both have to visit the desk to get your keys and show your licenses, that may be easier. But then there's not as much Preferred benefits, is there?