null
Clemens
Vasters: That'd truly be silly, wouldn't it? [Executive
summary: Those who only look at wire formats and are obsessed with
loose coupling shall look at some more complex and real-world
scenarios once in a while] Hmmm. I wonder who
that might be directed at? 
FYI: tomorrow, I will have been at IBM for 21 years. I spent nearly a decade working on AD/Cycle Information Model and Repository tasks. One customer I worked with during that period you may have heard of: GAD.
As to your example, there is no question that two "customer" entites that are semantically different should be modelled separately. The question to ponder is whether or not "ALTER TABLE table_name ADD column_name datatype" should be allowed and what the implications of this are.