Grazie Diego!
Fongu already helped me out with a quite simple solution that worked fine:
Using the performance file I had to set the parameters like that:
[WheelTextures]
Team #00=2,2
Team #01=2,2
Team #02=2,2
Team #03=2,2
Team #04=2,2
Team #05=2,2
Team #06=2,2
Team #07=2,3Team #08=2,2
Team #09=2,2
Team #10=2,2
So Team #07 will have two different sidewall textures for each car and that's exactly what I wanted.
Works also for [TreadTextures]
Great community where you can get solutions for almost every GP4 problem...