I don't think we're saying that they can't be done together and should never be done together. It's just that in certain circumstances it would seem better to have them apart. If you're talking economies of scale, when you go to, for example, order-making powers, then you're talking about expanding two offices.
It depends on where your values lie and to the extent that you have, in any given situation, a Cadillac or a Civic, I suppose.