There was only one sceen with both Brad and Julia in it

There was more than one scene they had together.
Granted, they were separated for most of the movie, but they did have a number of scenes together.
Spoilers...










In the beginning, there was the scene when they woke up together, then the scene when she threw his stuff from the balcony.
Then they separated, and didn't meet again until he picked Sam and Leroy up at the airport and the three of them rode in that car together until Jerry crashed the car because Sam said Naugahyde. There's the scene at the airport when she asks him the "when is enough enough" question and he answers just right. Then there's that scene in the hotel lobby when they're waiting for the phone call from the embassy, the scene up in their hotel room when she takes care of the pistol and hides, and then again when she comes out of the trunk of the car and he puts that ring on her finger.

Dogs are sons of bitches.
Rumrunner. Period.

The film itself was never presented as a star vehicle for them and was mislead as one in marketing, the attachment of the two stars was a real screw up in marketing cause stars usually make marketing easier, just their presence in the film is often enough to get people to watch but it can also create problems, in this case The Mexican was meant to be what it is even with Pitt and Roberts they still felt it was that way but when they started marketing it was a different story.