For Everest VR, you'll be able to redeem that and keep the app. However, once you've chosen your five apps for Viveport Subscription, you can't alter those choices until the end of your first month (AKA free trial). After that you can change every billing period (a month). So, I would suggest you wait until the end of your first month and then apply the Everest VR code after that. You'll be able to keep it then. I realize that's a little awkward, but you just uncovered a scenario we hadn't thought of!
In addition, I'm afraid every customer is only entitled to one free trial of Viveport Subscription, for one month. The one offered on purchase is the same as the usual trial. (Good assumption but wrong, @bulloko. )