STO between 2 plants

I am trying to configure STO between 2 different plants of 2 different company codes.

I have plant A and B which belongs to company code c1 and c2. We procure the same material from A to B and also from B to A. A to B works fine and i can do the GR and recieve it.

I cannot do B - A in the same system and i am geting the error

Customer xxxxxxxxxx does not exist (please change entry in plant xxx)

Is it possible in standard sap to configure both directions A - B and B - A?

If so, why am i getting this error?

Help apprecaited.

Sylvia