Mix records with a Ruby or Sapphire that has an Eon ticket, and Emerald will get the Eon Ticket. Use it at the SS Tidal to go to Southern Island, after the E4. You'll find a forest area that has an egg in the middle. Press A at the egg to find the Lati@s you did not choose. ex. If you said you saw a Blue Pokemon on the TV(Latios) you will encounter Latias, and vice versa.

Unlockable How to Unlock
Eon Ticket Mix records with an Ruby or Sapphire game that has one.