My understanding is the second injectors used in Direct Injected engines is to avoid the carbon build up suffered by the 100% direct injected engines. Carbon builds up like a Tee Pee on the back of the intake valves in DI engines. Dexos oil helps avoid that but it still build s up and you end up needing to walnut shell blast the valves at around 100,000 miles. Tradition injection sprays fuel into the intake charge which washes the intake valves as you run avoiding the build up issue. DI engines spray direct into the chamber and you have the carbon build up issue. Having both types injection avoids this. I have had GM, VW and a Mercedes apart with over 100,000 miles on DI engines and they all suffered the carbon build up issue.
Maybe the second injector enhances performance also, I have read that some of the Audi, GM and Porsche engines do run the second std injector in high demand situations to make more power. Some Turbo engines are doing this. I don't know these newer engines well so still trying to figure out the newer stuff.
The Direct injected engines are not compressing air and fuel, fuel is not introduced until the piston is very near TDC so detonation is not an issue and Timing can be far more aggressive and with std fuel injection.
I should ask my son to explain it to me, he tunes some of the Race Audi and Porsche Turbo engines which run both type injection systems plus turbos. His work is on engines with 600-1300 HP out of 3.8 V6's or 4.2L V8's. He laughs at my 6.2 with only 400 HP.
I bought CA gas this week, 3.85 for regular, premium was 4.45 or so, I may be off 5 cents on premium.