you'll need to have a shop take care of this for you... since there currently are NOT any direct bolt-on dual exhaust systems for the celica, you will need to have one created.
pro-shops have the welding tools to take car of this for you.
there are a few routes you can take care of this by...
1) purchase a dual exhaust system and have the shop custom pipe it to fit your Celica setup.
2) purchase two exhaust systems and have a shop weld things together for you (this takes skillz though)
3) forget about the cat-back and just go straight-pipe... this also requires custom piping - but you can have the exhaust come out where you'd like... center mount, rear factory locations... or even on the driver/passenger sides before your wheels
sorry for the poor photo collage skills - but I have been busy working the equivalent of 6 full-time jobs over the past 4 days