Is it possible to book a multi-class ticket on Delta, either online or over the phone? I'd like to book the following itinerary:

SLC-BOS in Y
BOS-LHR in C
LHR-BOS in C
BOS-NYC in Y
NYC-SLC in Y

There are great Biz fares on the BOS-LHR route, but I can't get them out of SLC. So, I'd like to fly coach SLC-BOS and then have Biz over the Atlantic. I'd book two separate tickets, but it would be nice to have them all on one reservation in case something happens along the way and I need some sort of reroute (almost happened to me once in St. Louis where I would've missed my connection in LAX to SYD -- a little nerve-wracking). I asked the agent when I checked in at the Marine Terminal last night, but she didn't know for sure and I was in a hurry.