Actually that was done before , a handful of games overlapped blended quads . Best example I remember was skies of arcadia in the Opening scenes , they overlapped a quad with varying degrees transparency per each vertex to give it a soft light coming from an angle but it wasn't the only time used on dc. Overworks again used that same effect on the intro stages of Shinobi 2002 ps2, so it's a pretty simple but effective technique.
The guy who did soul calibur 2 test for dc mentioned that 60 fps dc is more delicate performance wise than it is at 30 fps. Also mentioned at 30 fps layered transparencies become negligible to impact performance. At 30 fps you can have scenes that range up 60k polygons per frame and 2 to 4 layers of transparent quad with almost no performance cost with them on or off. So imagine if they had been less 60 fps obsessed and more detail oriented at 30 fps since it has more performance legroom at that rate.
Fake bloom skies of arcadia