Netsted ATO's, Rules to Compare across nested models

 I am going to set up a nested ATO in which I will have 2 other ATO's.  Let's call the top level ATO0 and the two nested ATO's, ATO1 and ATO2.  What I want to do is compare the number of selections within an option class of ATO1 to the number of selections with a similar option class of ATO2.  I want to make sure that they are the same number.  How do I go about writing this rule?
For example, if I choose 5 options in the ATO1 option class I want to insure that 5 options are selected from ATO2's option class before making a valid configuration.