I don't see why you need an renderbuffer or FBO in the first place. You're shadows are supposed to be put on the screen, right? If you're not using shadow maps (which I believe you aren't since you're projecting shadows to the floor, right?) then why would you want to render shadows to a different buffer only to read it back to put it on the screen again? All this can be accomplished by basic polygons, depth testing and a small depth bias.
Basically I image my floor like a big matrix, based on tiles. I need to know which tales have shadow and how much.
The FBO is mandatory since with the default FBO on the screen I'd not be able to manage the results.
If I however misunderstood you, I'd recommend that you keep only 1 renderbuffer and accumulate the shadows on the screen instead. Also notice that if you want to create a real-time application (e.g. a game) you pretty much can't use readPixels or any kind of readback from the GPU, since it kills performance.
Let's say that this operation of merging the shadows is not supposed to be frequent at all, but since the numbers in play are big I need to use the vga to avoid hours that a cpu would require. (and also because a rasterization is pretty easy and fast since is dedicated hw).
Only a problem is arising, at the end of the shadow merge, I'd need actually to render this final result on the floor... I really hope it is not a problem render to texture from a renderBuffer.. ^^
I still don't understand it. Either you do shadow maps with a FBO with a depth texture, or you can just render the shadow geometry directly to the screen. I still think you're doing this all wrong.
Anyway, you're supposed to use renderbuffers when you only want basic access to the pixels later through blitting, or you use a texture in which case you can feed it to a shader and do whatever you want with it. In your case, you could use a blit ( = copy) of the renderbuffer to the screen, but you'd need to enable additive blending to be able to "stack" lights.
Again, you're doing this wrong. Read up more on shadows and lights before you continue!!!