#1 and #2 fall in love and get married.  They each have their own health coverage already, but decide to cover each other to reduce their out-of-pocket costs when receiving services.  The first NAIC rule is subscriber/dependent, which means the plan that you are the subscriber to is your primary coverage and the plan that you are the dependent is your secondary coverage.  So for #1, #1s plan is primary and #2s plan is secondary.  And vice versa for #2, #2s plan is primary and #1s plan is secondary.  
A year later, #1 and #2 have a child, #3, and both decide to cover #3.  The NAIC rule for children of married parents is birthday rule, which means the parent who has the earlier birthday month and day (not year) is the primary plan.  [Ask #1 and #2 for their birth month and day] So for #3, __s plan is primary and __s plan is secondary. 
A few years later, #1 and #2 get divorced [you can make up a funny reason why].  Both #1 and #2 still decide to cover #3.  However, since #3s parents are now divorced, the NAIC rule birthday rule is no longer used and reverts to the divorce decree, which states which parent is responsible for the childs health care.  In #3s case, # [select person who used to be secondary] is listed as the parent responsible, so for services #3 receives after the divorce, __s plan is primary and __s plan is secondary. 
