To use multiple plans you will subscribe first to the plan you wish to do. To add a second plan you will then switch go to the discover page and choose a second plan - now both plans will show up, one on 'Active Plans' and the other one on 'Previous Plans' and you can switch back and forth between them at the bottom of your profile.
If you need help choosing a Fitplan just send a message to the Fitplan team through the app or send an email to support@fitplanapp.com.