Can you catch both Groudon and kyogre in emerald? - Answers

After beating the Elite Four and champion, you go to the Weather Institute outside Fortree City. Talk to the man inside and he'll tell you that there is an unusual amount of rain somewhere in the region. You travel there (you'll need to surf to find it), find a dark spot in the water that wasn't there before, use dive, and you'll enter Marine Cave. That's where Kyogre is. Once you either catch or defeat Kyogre, go back to the Weather Institute and talk to the man again. He'll describe somewhere with an unusual amount of sunlight. You travel to that area, find a cave entrance that wasn't there before and enter Terra Cave, where you can battle Groudon.