Usually, the second injectors (the ones far away from the valve) will only open at WOT and at high rpm. When this two conditions are met, airflow is really high and the sprayed fuel outside the trumpet will be sucked in with no problem.

Injecting fuel far away from the valve will cold down ACT's better and the air mixture will be more homogeneous which will help since problems can arise (fuel droping out of suspension) when loads of fuel is injected.

All F1 engines have injectors like this, and the "cloud" made by the spray on them is massive! No fuel spills out the trumpet.

Actually there is also other reason:
- When the injector near the inlet valve is too big, it will hurt low airflow mix (fuel will drop out of suspension and the injector injects too much fuel at low ms openings). Use smaller injectors and get another set far away to only help high rpm fueling.

It all dependes were the extra set of 4 injectors are placed on the inlet. You can have 2 injectors spraying at the same time right at the back of the inlet valve(s), but this would only be aceptable if the injectores were really small and top-end will hurt on the mix due to the two injectors not flowing enough.

Only if you go above 500cc or so that you'll start having problems with low airflow speed and small injector cycles.
Ok, this is just a rule from where to start because there's alot more involved than just saying "500cc is the max for any kind of inlet manifold design".

This aplies to any engine, NA or turbo charged/supercharged.