CHRISTINA AGUILERA & JESSICA SIMPSON PARTY LIKE IT'S 1999!!

Former rival pop stars Jessica Simpson and Christina Aguilera had a chance to reignite their battle during an epic party ... but only one of 'em made it to the stage. 

The singers and their husbands hit up Linda Perry's L.A. party to celebrate the music she wrote for the new movie "Freeheld" -- and it turned into a major jam session. 

Christina performed a bluesy cover of John Lennon's "Mother." Juliette Lewis, Courtney Love and Perry also took the mic. Even Jess' lil sis, Ashlee Simpson, performed.

Jessica did NOT sing, and instead chose to just chill at the event. Props to her though ... she did not need assistance leaving the party this time.

"Invest in the brand!"