One house, 2 different Sonos systems, 1 app

Sure, just set it up as a different "house" or "System", or whatever term they use. Don't add it to the existing stuff. There will be the slight difficulty of having to choose which system you connect to when you open the app, and the parents will be able to do the same, but there's no reason why it can't exist on a separate "home". Not sure why you'd need to do so, though. Can't you just set it up as a different room on their system, and use it that way? It's not like you can't trust your parents to mess around with it, is it? Or are you concerned that they'll start waking you up early in the morning with a brass band? 🙂

Is there anyway to control 2 systems from one app when they are on the same wifi?
Not really.

Attempting to use the same controller to access two Sonos systems/'households' on the same network is pretty impractical. The controller is registered to both systems, so when it starts it could pick either of the systems to latch onto. It all depends on which player is the first to respond to the discovery broadcast. Results will be frustratingly unpredictable.

You'd need to dedicate a controller to the PLAY:1's own system.

I've never tried to do 2 systems in one house, I'd have thought there was a "picker" involved, much like my Harmony app does. I do have connections to 3 different Sonos systems, but it's in 3 separate homes.

Thanks for the clarification, ratty.

As I indicated before, I'd just set it up as a different room, and rely on the parent's discretion to not mess with your "stuff".