Machine Gun Kelly, Yungblud Jump on Tour Bus Roof, Perform Outside The Roxy

Machine Gun Kelly and Yungblud joined forces after separate gigs in different cities ... with an impromptu performance atop a tour bus that had fans losing their damn minds.

So check it out ... MGK had a scheduled performance Wednesday night at the City National Grove in Anaheim while Yungblud was performing at world famous Roxy Theatre in WeHo. But, just after midnight ... MGK rolled up on his tour bus, spotted YB and, in endearing fashion, yelled, "What's up, you f***? Come here."

Halsey's man didn't hesitate joining MGK on the bus rooftop and quasi performed their new hit "I Think I'm OKAY" featuring Travis Barker. The crowd ate it up. One problem ... the duo had no idea how to get the hell down. Lucky for them ... the crowd had an easy solution.

BTW ... MGK and Yungblud sauntered over to On The Rox ... joining Travis to perform their hit track. MGK was hanging all over the place Yungblud sprayed champagne everywhere.

Super BTW ... we ask MGK if the new tattoo on the back of his head -- which reads "Hotel Diablo," the name of his new album -- hurt at all. Ya might -- or might not -- be surprised by his answer.