Doing a fair amount of business travel between two cities, I am always monitoring the fares. When the sales are on, I book two or more trips at a time. The travel agency my company uses has a negotiated discount with CO and NW, roughly 21%. The TA will not book back-to-back tickets because it is against the airlines' "law".

Wanting to stay "legal", I book one trip A to B to A and another B to A to B, using a different carrier for each ticket. But, as it turns out, the CO flight is actually on NW metal. So I get 2 r/t tickets, back to back, all on NW and all legal. Isn't that interesting?

And if I really have the time to kill, I check the fares on each combination because sometimes a r/t on NW but using CO co-share numbers yields a lower fare.