you should use the query property of the second combo it should look something like this
comboBoxDigit.on('change', function (evt) {
var CmbCustomerAttribute = registry.byId("Attributes");
CmbCustomerAttribute.set("query", {filteringProperty: this.get('value')}); //if you dont understand this check out the stores tutorials
});
EDIT: tutorials referenced in above code snippet helps clear up the ambiguity of "filteringProperty" quite a bit.
I would also recomend to populate your sencond combo from the beggining with something like this
var CmbCustomerAttribute = registry.byId("Attributes");
CmbCustomerAttribute.set("store", store);
CmbCustomerAttribute.set("query", {id:'nonExistantId'}); //so nothing shows in the combo
I think somthing like this is what you are seraching for, any doubts just ask. as cant say mucho more whitout a fiddle or actual code