AA's Price & Schedule also allows mix-and-match for each direction, but still requires, in the end, a RT purchase. Certainly possible that it allows actual purchase of the OW separately, and it really is half the RT price. Hasn't been the case in the admittedly few cases I've looked at, though.
It would be interesting to see some concrete examples.
Cheers.
For example, Labor Day weekend travel Sep3-Sept6 non-stop BOS-LAX prices out the lowest at $386.40 round-trip.
Or you can book BOS-LAX on Sep3 as one-way for $179.70.
Then add LAX-BOS on Sep6 as one-way for $206.70.
And no - it doesn't require RT purchase. You can purchase each leg as a separate ticket, and I've made use of this recently where it's cheaper to buy the outbound leg for cash but use a 1-way award on return.
The price comes out the same whether the itinierary is purchased as single RT ticket or two OW tickets. And this is the lowest fare for the city pair.
This is not to say that some fares aren't still priced lower for RT vs. OW, but there are
increasing number of fares which are priced the same for OW/RT.