To take multiple plans on iOS, you will subscribe first to the plan you wish to do, then switch to the second one in the discover page - 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. Here's a video that better further explains how to do it:
We are working on adding the same feature to Android. For the time being, you can change your Fitplan at any time - all you have to do is subscribe to a new plan (note that this will reset your progress in your previous plan).
You can also supplement your Fitplan with single day workouts in the FREE/FREESTYLE section of the app.
If you need help choosing a single day workout just send a message via the chat icon in the lower right corner, message the Fitplan team through the app or send an email to firstname.lastname@example.org.