You have to use a shader for this (for instance the diffuse lighting shader from the online tutorial -> shader workshops), then write CullMode = None; in the pass before the shader compile instructions.
"Falls das Resultat nicht einfach nur dermassen gut aussieht, sollten Sie nochmal von vorn anfangen..." - Manual
Well, setting CullMode = None; in the Diffuse shader worked good. I had to set the OVERLAY flag for it to sort correctly. Is that the correct way to do it? Or does anyone know of a better way?
Thanks Malice! I'll try that way too.
Edit: I have A7 Commercial which seems to not have the double sided material.