Balkie-As Hunter said, get a few half bottles(not "splits"-those are 187ml), pour the first half of the 750ml into the half bottle, almost all the way to the top. Take the cork out of the newly opened bottle(that way, you know what's in the 375ml two weeks later) & push it in until wine begins to squeeze out, so there is NO air. Put it back in the fridge or wine cooler. Whenever you open it(even a month later), it tastes like it has had about 10 minutes of air.
Forget argon or nitrogen gas in the half-empty 750ml bottle. I find it to be worthless, unless you're drinking the rest the next day. If you're really anal, you could flush the 375ml with gas BEFORE you pour the first portion from the 750ml.