This isn't directly related to accumulating mileage but haven't found any information related to it elsewhere:

Which airlines allow booking roundtrip in multiple classes?
ex: LX allows me to book my overnight JFK-GVA in P (business) and the daytime GVA-JFK in T (economy).

I've found that I can assemble this type of itinerary on Swiss and United websites and imagine that other carriers must allow it also. Would one have to call and make a booking to combine seat classes?