Ok, a turbo is spooled by two things: exhaust flow, and heat energy.

Now, [this is all theorhetically speaking, I don't know who may have ever tried this] a smaller primary leading to the turbine would create more backpressure and heat at lower RPM, so it would be possible to spool the turbo early. By switching over at some later RPM value though, you risk cutting away some of that heat energy, or staying with the smaller pipe too long and limiting how much boost can be generated.

Then there is the issue of why you'd want boost earlier than the ~3200-3600RPM a t3/t4 would spool on an H22 anyways, since it would take some serious traction to keep from wasting that torque at low speeds.

Also take into account the longer distance between the exhaust ports and the turbo you'd have to have to accomodate a 'y' pipe, and I start getting the feeling that the potential losses far outweigh the slim potential gains.