i have seen this question before and i can garuntee that it is a bad, bad idea to put 2 different sized trubos into one car. when the larger turbo starts spooling it will have more back pressure on the smaller one and cause it to drastically slow down and maybe even stop which puts way to much strain on the first one and it could blow. by the way, a T60 and a T66 together would make WAY over 700hp because the T60 on its own makes about 500. it is possible to put to of the same sized single kits in the car but you should use small turbos because they would make way to much hp if you use big ones. to do this i would recomend the T04S04 line of turbos like saad saad. you can use the .58 A/R housing and build about 800 hp.