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Gerard Butler, John Mayer

Paul Redmond/Getty Images; Jason Merritt/Getty Images

Look out, ladies!

Single dudes Gerard Butler and John Mayer teamed up musically this weekend in NYC during Gerry's 41st birthday bash at the Darby, a source confirms to me.

First, the birthday boy sang Oasis's "Wonderwall" with John on guitar, then the men did a rendition of "Mustang Sally" together.

And even though Jennifer Aniston (with whom they've both been linked) wasn't at the soiree, her name was dropped that night...

According to Page Six, Aussie comic James Smith joked that Gerry and Bounty Hunter costar Jen had "as much sexual chemistry as Regis [Philbin] and Kelly [Ripa]."

Other famous faces actually in attendance included Jay-Z, Kate Hudson, Adrien Brody and Bono.

Nice turnout, huh?

No word on just how John and Gerry met, but a source tells me "they've been friendly for a while."

We smell a wingman match made in heaven!