No, there's a problem with the user not reading how to use it properly... As it says on the page, you need to do it sector by sector. BA don't fly to MSY so at a guess you'll need to calculate LHR-DFW in WT+ and then DFW-MSY in AA Economy and add the two together (unless you're doing a different route but IIRC last time I flew it, DFW was the default gateway offered to me).