I think someone already mentioned the name of an aircraft, but I have to agree. Sally was the nickname given to a Japanese Bomber used in WW2 (Actual name being "Mitsubishi Ki-21), and bringing Zombies into this, Mustang is the name of a U.S. Bomber used in WW2 (P-51 Mustang). And old COD titles generally focused on U.S. and Japan also against each other, (like in World at War) so it would make sense for them to make U.S./Japanese names paired together. And the idea that in Zombies, Mustang and Sally fire grenades instead of bullets further supports the idea (even though Mustang and Sally weren't around until Black Ops). Also, (according to the COD Wiki) Mustang and Sally is apparently a reference to a song by Wilson Pickett named "Mustang Sally". Oh and something I actually found interesting that is more of a coincidence than anything, is that in "Saving Private Ryan" (which Treyarch did actually reference a lot in games; most notable is World at War) at the end, a man shoots an M1911 at a tank and it suddenly explodes. It turns out that what blew it up was actually a P-51 Mustang. Most likely that's not where the name originates from, but it still is interesting.
