Interesting, I am drawing a blank when a subscriber will want to purchase two subscriptions on the same website... ? Is he or she purchasing the additional subscription for someone else?
Yes, but ultimately you'd probably want to bill them all in one go every month, because otherwise you're paying far more in card processing fees than you need to.
We needed this for our site. You can add as many 'domains' as you like, each with their own plan. Instead of creating a new customer + subscription we hacked together a way that each customer gets a unique 'plan', which can be multiple subscriptions. Multiple subscriptions per customer would have saved us a lot of time.
The other option was asking the user for CC details each time and creating a new customer at Stripe - but that felt like friction.
Your customers could have different levels of subscriptions for all kinds of things: CPU ('dynos'), databases, backups, monitoring services, etc