IR ISO Across OU

HI, Im little bit confused about the CUstomer and internal location association. im in R12 and i went to Customer and selected the location for OU2 and assigned the Internal Location. And i went back to customer and selected the location for OU1 and when i try to assign the same internal location. It errors out "Internal location is already assigned to another ship to address. Please choose a different location." Please advise But it