In ruby, sapphire etc? Only one of them appears in each version. They appear randomly, I think you can actually see one on your map. Unless I'm thinking about D/P. There are supposed to be certain places the pokemon will go to a few times, and there's supposed to be a list of places they are found most of the time. If you look online, you should find something similar.
I haven't played my Ruby in years, I caught Latios, but I''m pretty sure a little icon appeared on your map, unless my memory got a bit switched with my memory from playing Diamond. It depends on what answer you said, red or blue. Then you can get the sea chart thing to get the other one. Here's a strategy that seems to work well in Ruby. Might work in Emerald also. http://www.neoseeker.com/forums/3271/t644124-great-strategy-to-catch-latios-latias-have/
Then what's the point asking?? Anyway, on the site I mentioned, supercheats, there is a warp code available that will take you to latias/latios location, mew, ho-oh, lugia & deoxys' location as well