My thoughts on this are if you sell the turbo that came with your CA you will only be $600 - $700 short of buying a S15 turbo. This would be the best budget option short of spending the $ 2600 on the big sucker garrett range.
My reasons are :
1.The S15 turbo will bolt onto your manifold
2.The S15 turbo will more than likely have less kilometres and be in better condition than most T25's
3. S15 Turbo is ball bearing so it spools up fast for minimal lag.
4. You will have to find a T25 of similar condition to use
5. The S15 turbo is ball bearing and is capable of 200rwkw+ on a S15
6. I assume this is a CA18 in a 1200, you may struggle to fit them between and around the strut tower.
7. You will need to fit two exhaust dump pipes then into one large pipe in the bay before the exhaust levels out and travels the length of the car, this would need to be done to clear the steering and the road. This could be done Toyota soarer style to reduce room needed, i guess.
8. Twice the plumbing no real disadvantage just a pain in the ass.
9. Lots of lag, by using two turbos that is alot of air to push, basically a T25 you are running a GTR sized set up on a 1.8. It would be laggy but you could always try and drive around it.
10. The money spent on extra piping, exhausts and clamps etc, will be similar money it would of cost to by the S15 turbo in the first place with a lot less brain damage.
They are just my opinions and 10 things i could think off the top of my head. It may be cheaper in intial outlay but more expensive in the long run. I ran this subject by Vince Rigoli not that long ago for another project he had in mind and he replied "twin turbo, twice the trouble".
Although if your budget can stretch to the bigger garrett range (like on Dattodude's CA in his avatar) then that would be the best option.