If you got the Surf HM on the last floor of Solar Cave, then the Latios & Latias event should trigger (unless you haven't yet purchased the Southern Island Friend Area, in which case you SHOULD have). I've started a new game, cleared the story, beat Kyogre on the first encounter in the Stormy Sea, encountered the Regi trio & Mew, cleared the Buried Relic (FINALLY), have already purchased the Southern Island Friend Area when the post-game began, and just got the Surf HM from the Solar Cave (lucky I brought a Water Pokémon teammate with me and made it the leader to get it, otherwise I never would've picked it up), and I made both the Spinda and Latios & Latias events trigger!!!!! Basically, if you don't have the Surf HM, go to the final floor of Solar Cave, make sure you already brought a Water Pokémon with you as your partner, switch leadership to said Pokémon, grab the HM, switch back to the previous leader, leave the dungeon, and BOOM!!!!!