Jennifer Lopez / Enrique Iglesias Concert -- Who's On Top?

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Now that Jennifer Lopez and Enrique Iglesias have announced their concert tour, the big issue is .... who gets top billing?

Lopez' manager, Benny Medina, tells TMZ, "Neither Jennifer nor Enrique care who opens or closes. What we want to do is give the fans the best show possible and whatever that means, that's what we will do."

It's a big change from a year ago, when Enrique bailed on the Britney Spears concert tour because she was getting top billing ... and he wasn't having it.

We're told the NEW tour will be a "co-headliner show," with equal production value, money and time spent on both Jennifer and Enrique.

As for who opens and who follows, Medina tells TMZ it hasn't been worked out yet but it won't be a problem. He also said if the venue has a bunch of screaming girls pining for Enrique, it would make total sense for Jennifer to go first.

An AEG rep involved in the concert tells us the plan is to have the two singers flip flop, with Jennifer going first one night and Enrique going first the next.

For the record -- the AEG rep also told us Enrique had "no ego" about going last.