I'm no tire expert, so take what I say with a grain of salt...
From what I read and understand, it is okay to buy just two new tires as long you have the exact same tire on the same 'axle'.
If I remember right, you want to put the new tires on the rear. Why?...you want the rear to have most traction to prevent oversteer. (Apparently, understeer is better to have than oversteer.)
With VSC/ATRAC (and possibly 4wd), I'm not sure that matters any more.
I believe it is safer to have all 4 tires be the same type...all season, snow tires, summer tires, etc. For maximum safety, I think it is better to have all 4 tires have the same amount of potential traction.
But I have hear of people buying just two dedicated snow tires and putting them on the rear on a rwd vehicle with no real issues.
For a full time 4wd (not sure if yours is or not), it may not matter.
Well, not sure if I was much help...in any case, I have heard of people (and I've done it myself on a different vehicle) buy just two new tires...just make sure they are the same size as the ones you are going to keep.
The unknown factor here is ATRAC and VSC...will it get overloaded trying to compensate for the axle with lesser traction (in adverse conditions)?