Normally, no. After you defeat the Elite Four, Latias will be capturable once you check the TV to see a commercial saying something about "a red Pokemon flying through the land."
The other one, Latios, can be captured on Southern Island. You can reach Southern Island if you have the Eon Ticket (an e-reader card I think) which Nintendo gave out. You may or may not be able to buy it from stores such as GameStop. Or you can just cheat.
Simple Questions next time.