Finaly I was succesful after some googling. Sorry I did not it before asking my question, but I was confused by the example mentioned in the FAQ.
To provide feedback to others it was necessary to add the name of the column where the "join" is made.

class Customer(SQLObject):
    class sqlmeta:
        fromDatabase = True
        idName = "CUST_NO"
    sales = MultipleJoin('Sales', joinColumn='CUST_NO') # this is what I have to add to my code

class Sales(SQLObject):
    class sqlmeta:
        fromDatabase = True
        idName = "PO_NUMBER"
        idType= str
        custNo = ForeignKey('CUST_NO')

thanks and best regards

Petr Jakes