TT 4 cylinders are very doable, but not common because there aren't many (if any) kits out there to do so, and there isn't enough reason to offset the cost. Twin turbos have the benefit of better response and spool, and greater turbo life than singles, but are expense, and with most performance cars around here being only drag-focused, response isn't a factor. Also, the smaller the motor/turboes, the smaller the response benefits of a twin setup.

The cost (and extra weight, though mostly negligible) isn't worth it for most people, when they typically just have a simple HP goal, which can always be reached by a tried-and-true single, often available in a complete, cheap kit.

For the record, having two cylinder banks doesn't make it that much easier to have twins, as compared to an inline having twins, just makes it that much more sensible to have twins as opposed to a single on a multi-bank engine.