Adding a User as Contact User to several Suppliers

We have some External Users which need to have visibility to other Suppliers data (Item, Change Orders). The External User is mapped as Contact User to it's own Supplier, but when adding him as Contact User to another Supplier, he is actually moved between the Suppliers, not only added. Does anyone know how an External User can be mapped to several Suppliers?We are using only one Usergroup for External Suppliers, and restrict their visibility to data through the Supplier Contact User tab.