Does anybody know why (and which) certain US city pairings are valid routes for open jaw bookings online, but other pairings cannot be booked online? For instance, SYD-CMH//DFW-SYD is an invalid route that I cannot book online, but SYD-ORD//DFW-SYD (for instance) is perfectly valid. Woudl something like SYD-CMH//DFW-SYD be valie through the (charged) call centre?