Yeah I can speak to that. You are referring to TTC or "True Twin Conversion". Actually it decreases low end torque/power beause it causes both turbos to come on at once at about 4500rpm or so. One good thing is that there is no transition from the 1st to 2nd turbo seeing they both work in unison. In sequential mode, there is a split second delay before the second one charges up. You still run the same amount of boost with TTC, so it's really not worth it in my opinion. There is a joke about that setup because you get more lag and the same amount of power. Most people use TTC as a tool when troubleshooting boost problems with the sequential system. TTC is also a little safer than sequential..I've heard of people blowing their second turbo from overboosting if something is wrong with their sequential plumbing.
Personally I really enjoy the low-end response from the sequential system. The first turbo comes on line at 1,800rpm with full effect by 2,500rpm or so. It usually good for .7 bar on a BPU car and by 4500rpm the 1st turbo feeds the second and whoooosh you get the afterburner effect. I run about 16-18 psi on 91 pump. IMO max boost on stock twins should be no more than 18psi. You could run more with higher octane, but you are out of the efficiency range by than and your just pushing hot air.