First of all, just an intake won't do crap for your car. You need to do the following:
- Bigger headers
- Bigger downpipe
- Catback
- Cold air intake
This in all would give you about an 8hp increase tops, since a Vitz isn't exactly a performance car.
Secondly, a dual exhaust is a really bad idea if you actually connect both pipes because:
It may actually ruin your car's output because it's just a 4 cylinder vehicle. Once you divide the pressure you'll notice a damping of power, you'll actualy lose output on lower RPM.
Excessive noise and fuel consumption
If you just want looks, put in the second exhaust but don't connect it.
Lastly if you really want performance, you can always turbo the car or you can get a supercharger from TRD. Both are expensive solutions.