Hi -- trying to book this flight:

Outbound: DL 5 JFK to LHR on 12/25/13 in Business
Return: VS025 LHR to JFK on 1/3/14 in Upper Class

When I try to book this pair on Delta it comes out to $4,451 (vs. $2,354) if I booked DL flights both ways. I go over to VA and the same itinerary prices out at $4,040. The interesting thing is that on VA it shows me the pricing of the separate legs and the JFK-LHR leg on DL is showing $2,723 with the LHR-JFK leg on VA at only $1,317. This makes me think that if I try to mix the DL and VA legs on either website it is pricing out as two separate one way tickets. Is that right? This doesn't seem quite right to me. I thought the point of the alliance was to provide more seamless flight options. What's better about paying essentially two one way fares on one website? I could have just done that before (though granted not earned any SM on the VA leg).

I will probably just book RT on DL coded flights (so back on DL 2) but was hoping to be able to get that later flight back.