We’re married and her parents don’t know. My insurance doesn’t cover her treatment but her parents’ insurance plan for her will. Is it illegal to have health insurance for 1 person from 2 companies? And will her parents find out that she’s married with insurance if she uses their health insurance plan?

Naw it’s not illegal. The hospital/clinic will ask for her insurance and just tell them the one she’s on with her parents-they aren’t going to look anything up beyond that.
health-quotes.talk4fun.net – here is my health insurance plan. As I remember they can provide such a service.