As Megaswifter says, your TV choice decides which of the 2 you can catch. The other sadly, Is unobtainable without an event/trade.
When you do this, Latias or Latios is made available for capture. However, they can appear almost anywhere, in random encounters, and are very likely to flee. They behave similarly to the legendary beasts of Johto. Unfortunately (as far as I know) there is no way to track either of their locations, since they change randomly as you travel.
- They can even be encountered in Sea Routes while Surfing!