ok ok ok , I'll just break down the basic summary for what was discussed on the old forum.Twin T2's will flow like a small T4, this handed down to me from a couple guys at Turbonetics that did this with a 1.8L Integra and popped the headgasket. They spool up quick, but two force alot of air.
On a new T2, T3, or most any turbocharger with an internal wastegate, the external actuator arm has a 7psi spring in the vacuum canister. This means that once you boost up to 7psi, the boost signal in the line begins to defeat the spring that holds the actuator arm tight (which holds the wastegate shut tight), and the wastegate begins to lift and open.
With two T2 turbos, they will make that same 7psi in the manifold as a pair, but each will only be putting out about 3-4psi each to do so. This means it takes 5-6 psi each turbo to make 10psi in the manifold, and 7-8 psi in each turbo to make 15psi, 10 psi a piece to make 18-20, and so on. So you stay within the efficiency map of each turbocharger. Of course this explanation only works if both turbos are identical specs and boosting parallel (and not sequential.
However, I had an idea just now for a sequential setup using a T2 or T25 paired with a T3/T4. You can set the MBC on the T2 to 10psi, and then on the bigger turbo set it to whatever else (15, 20, 25...), by the time the T2 is running out of breath, the bigger one is now spooled and climbing. Hmm...
Just as in the case of the parallel setup, you would need a Y-split with the single path at the airflow meter end, and the split paths leading to the inlets at the turbos. Then another reversed Y, with the splits leading from the turbo's blow outlet and the single path connecting to an intercooler pipe.
That could work :RTFM:
...provided there is enough exhaust power.
If there isn't, then at least the parallel twin T2's will.