Ordering extra items within a subscription

So basically, we offer clothing boxes within a subscription. They always get 1 box/month. Within a 6-month subscription we offer 2 boxes or more (still 1 box at a time).

Is there any way of allowing the customer to order a second product within the same subscription? Having two subscriptions would mess with our email system and due date settings – so that’s sadly not a solution.

All help appreciated ??