It's no longer available from Amazon, except from private sellers. DVD Planet says it's discontinued. If you still want it and can find it, better get it before it skyrockets on Ebay.
This is the same thing that happened when Universal released the Cinderalla Man two disc and that's available again so I wouldn't worry too much if you can't find the two disc Jarhead right now. It'll probably happen with King Kong in a few weeks too.
Could it just be that the studio underestimated the demand for the 2-disc edition so that retailers are selling out and it will be available again as soon as they are restocked? It sounds like that's what happened with Cinderella Man and Kinsey.