The new Kayak feature of selecting flights individually inbound/outbound is cool, and very helpful.

I have a question however: On Travelocity I can select UA for outbound and say DL for return, with this kayak feature, it forces the return on the outbound carrier.

Does anyone know why Travelocity allows different carriers out and ret, but kayak does not. Obviously some fares are based on the return ticket, but I found quite a few on TL that are work either way.

Thanks.