Seems I found a rather weird solution for my problem. Since I couldn't find a way to combine the two layers with different patch sizes, I duplicated the plane and disabled one layer in the first one and the other layer in the second one. I didn't expect that to work, but surprisingly it did. I had to move the plane with the smaller patch size a tiny little bit downwards, because for some reason I don't know, the second plane only showed up half. Like split diagonally. No idea, why this works...
Unfortunately this almost doubles rendertimes.
I would have preferred another solution, but at least I got what I wanted.
Jordan, using some additional procedural foam patterns or even image maps doesn't look good, as far as I've tested it. Even when I use an input warp shader that's fed by the displacement via a redirect shader, the pattern's distortion doesn't really move in sync with the mesh's distortion.