But somehow it needs to work, because in my exhample multiple different cars use the "vehicles_muscle_n_w_interior.ytd". Otherwise creating shared textures doesn´t make any sense.
You can assign multiple cars to the vehicles_muscle_n_w_interior texture, you just cannot assign multiple cars to the polshare texture and assign this polshare texture to multiple interiors as well. Multiple children can have the same parent, but a child can only have one parent.
Also, regarding the ytd size: It is not a useful metric for memory usage, as ytds are compressed similarly to zip archives to decrease disc space, the size of the dds textures themselves is what matters. Because of the ytd compression, it is possible that smaller textures with more varying content can result in a higher ytd size than larger but more repetitive textures, even though the smaller textures require less memory.