Robert Downey Jr. & Mel Gibson: Double-Double Date

3/21/2011 3:25 PM PDT

Mel Gibson and Robert Downey Jr. -- Double-Double Date

Robert Downey Jr. and Mel Gibson got together for a friendly celebrity power lunch yesterday, but it didn't go down at one of those super-expensive Hollywood joints -- they got burgers at In-N-Out ... TMZ has learned. 

The old friends found themselves out in Irvine, CA when the hunger pangs hit -- so they went to every West Coasters favorite fast food spot. 

According to a witness, they each ordered a Double-Double combo meal and dined with a third person, non-celebrity. They were joined by a friend, who did the ordering for them.

BTW -- We know the pic is blurry, but we've confirmed  ... it's definitely them.

No word on why the two were in Irvine together, but we're told it definitely wasn't for a movie.