How can i use custom variables to chain

With reference to the above link, How can i achieve the following.
I have two variables say region and servers. How can i map regions to server.

region1: reg1-serv1, reg1-serv2
region2: reg2-serv1, reg2-serv2
region3: reg3-serv1, reg3-serv2

region: region1, region2, region3
servers // This variable is dependent on what region i select.

I was unable to achieve the same with custom variables. Also i was unable to chain custom variables.

Any help/example is appreciated.