After many "try and cry" attempts:
I succeeded in combining PETG and TPU and printing it with the MMU2S.
I made a smartphone flip case for a Samsung J7 and a Xiaomi Mi A2.
The outer layer is printed with PETG (turquoise), the case with TPU (semisoft, white).
The connecting bridge ist only 2 layers thick. The second layer is wider so the two filaments stick together perfectly.
The TPU and PETG stick together surprisingly well. I couldn't scratch one layer off the other.
I will add descriptions and infos about the settings in PrusaSlicer later. Loosing the idler was necessary...
And of course will add a gcode-Testfile (no use in printing it when you don't own a J7 and a A2...).
... tbc ...
The author hasn't provided the model origin yet.