It might even be considered international and you can use the RCC/MLL if your in first.

Like qasr said as long as UA issues the ticket it should be fine, but you might get some confused CSRs who might not issue it. If they don't ask them if they publish a tariff for the routing, if UA does you can use it. However, you can not use this if AC operates a segment, that would be known as cabatage or something like that. No carrier in North America has the rights to do that.