Hello Pete,
You can also use Geography boundaries as follows:
Go to Define Areas > Select Geography > Select from the map > select the desired intersecting geographies > you can choose to merge these geographies into one > then click Options and select No fill > Ok.
Then create your CCM and use this boundary as an extent. For example, here I created a CCM of all the counties in AZ showing a total population.
Hope this helps,
Fasil T.
Business Analyst Team