Drop-down list choices restricting choices of other lists?

Hi,

I used to think I was quite capable of handling Excel, but as they say, ignorance is a bliss.

Title explains my problem quite well and furious googling gave me an example which lets me explain what I'm trying to do even better.

http://www.contextures.com/xlDataVal13.html

Behind the link you can see, how for example choosing Ontario as a region, lets you choose only certain customers.
But is it possible to make it so, that certain customers would be available for two or more regions? For example, customer Mel would be available for both Ontario and Alberta?

Ofcourse, you could do this by creating Mel for both Ontario and Alberta, but when dealing with huge quantities of data, this method isn't an option.

Is this possible to do at all? What is the best way?

I'm grateful for all kinds of tips and help, thanks!