Solution...finally. In this code example, myDataColumn is a field name in my data store--the same field that I use as my ComboBox display value.
// I tell my second ComboBox that whenever you load,
// select your first value automatically.
// I load my first ComboBox from the server.
// I tell my first ComboBox that when the user selects a value,
// reset and reload the second ComboBox.
// Oh, and pass the value of the first combo to the script
// that loads the second combo. This way, we only load
// values that are applicable.