First of all, thank you for your answer
Secondly, your code does work - but it's just what I already have. I think even with (or because?) your Dr. Pepper and Epiphany you misunderstood me.
The problem I'm stuck to is: A Sprite on Layer 2 can collide with a Tile on Layer 1. That wouldn't be a problem yet. Now since the whole map is parallax-scrolling, I have the problem that the coordinates (50,50) on Layer 1 aren't at the same place as the coordinates (50,50) on Layer 2, so I have to calculate which pixels on Layer 1 are just below/in front of the Sprite on Layer 2. The whole thing is kinda 3D. Well, almost.