Fans at Britney Spears' Femme Fatale concert in Atlantic City got a two for one deal ... Ms. Spears AND Lady Gaga!

During the show, Brit announced to the crowd that a "special guest" was in the audience.

Considering the "Slave 4 U" singer pretty much lip-syncs her way through the show, wonder what Gaga thought of that ... since the "You and I" singer favors singing live?

Watch the crowds reaction to seeing Gaga in the above video. Loves it!