Your big turbo is not always your limiting factor. If you are running a small primary turbo, you will have a hard time. Remember, you have to force all of that air through the small from the big one. When it comes to exhaust you need to flow better also. If you have a small turbo and housing for the primary, it is going to restrict it. Now some people say they can get a 3b to spool as fast as twins. That might be possible and more efficient than twins. If you do run twins, make sure you have a big wastegate on your primary feeding the secondary turbo.