Check the web page of your insurance company

to see if they have the "NON-DUPLICATION OF BENEFIT" clause. I bet they have. It means that suppose you have two dental plans: the primary plan pays $2000 and the secondary pays $2000 or less for the orthodontic work; you'll get $2000 from the primary plan, and $0 from the secondary plan. However, if the secondary pays $3000 for the ortho work, you'll get $1000 from the secondary plan. The secondary plan always pays the difference between its own benefit and the amount already paid by the primary plan.