Don't get too confused here, two of the
same size turbos compared to only one of the
exact same size turbo will indeed spool slower.
However, two
smaller turbos that effectively equal the size (when added together) of one larger turbo will actually spool a little
faster than the single turbo, because the moment of inertia (MOI) increases at the square of speed, and these things spin
fast. The two smaller sets of wheels have a lower MOI than the one larger.
So in short, each turbo is receiving half of the exhaust, but they're half the size (of the single), but they still spool faster because the smaller wheels have less momentum to build up.
In the real world it isn't a huge spool difference, and packaging is more of a concern. A single is simpler and so has less to go potentially wrong. If sized right, it will work perfectly well. That's why STS decided to go with the single rather than dual for the LLT (and I think they might not have small enough turbos to make them usable anyway. They already had to downsize the exhaust housing as it is.)
Hopefully this clears up any misconceptions.
(BTW, yes, I am still watching this and the other turbo threads.
)