Here are a few ideas for the tour poster:
An open road theme with both artists hitch hiking down a long empty highway with a nice desert sunset.
Since its a tour with 2 headliners I thought it could be cool to do a more tongue-in-cheek poster of a cow with two heads!
The guitar case would be covered in vintage inspired travel stickers - one for each date on the tour. The typography could look like an old sign painted on the brick wall in the back.