You can book a split economy/BE itinerary online if you find the available fare classes upfront and then enter exactly what you want using the method described here: (Untested) Guide for Complex, Online Multi-Carrier Bookings on 006/DL

For example, for an MSP-LHR round-trip wanting economy for a SEP 02, 2014 outbound and a SEP 04, 2014 return in BE on the nonstop flights, I did a quick search for that itinerary as round trip in both cabins individually. I noted the lowest fare class offered for SEP 02 in economy (which was B) and SEP 04 in BE (which was D), then used those as the fare classes in my manually created search.

After following the steps in the thread referenced above, here was the result:



Edit: I should add that I have obviously not called DL to verify that this is the same price a phone agent would quote for the same flights, but it should be.