People don't do it because it's very hard to do properly, even the excellent HKS twincharger kit has bodge aspects and is very hard to tune, and the benefits are dubious. A computer with programmable outputs, such as a MoTeC is mandatory for a custom system as there is no point in having both the turbo and supercharger blowing at the same time. The idea is to start with the supercharger and transition to the turbo once the latter has reached full boost. To do this properly requires very tricky plumbing, a clutch for the supercharger and a lot of tuning to get the crossover right.
The theoretical advantage of twincharging is reduced lag as the supercharger theoretically has a better response than a turbo, in reality though even this is not really true.
In terms of outright power the turbocharger wins hands down, and a properly tuned turbo only system is better than a twincharger system for all sorts of reasons. Cost, practicality, durability etc. etc.
As an example there was an article twincharged Treuno featured in NZ Perfomance Car magazine a couple of years ago; take it from me that it never ever went past 5000rpm or so because Mr Greedy couldn't get the SCer and the turbo to work sequentially, and the 4AGZE is not really up to 30psi of superheated air . The owner eventally despaired, pulled all the gear off the car and sold it.
Cheers
KiwiMR2
