You will first want to make sure both subscriptions are active in your account. If you are already an active subscriber to one of the boxes, you can always add a secondary subscription within your account without subscribing to a secondary subscription on the store front.
Once logged in, click Manage Subscriptions. Select a Subscriptions tab, then click Add product. This will open a pop-up where you can search, view, and select additional products to add to your upcoming deliveries. You may also choose the frequency and first shipment date.
To rotate between subscriptions, once logged in, click Manage Subscriptions. Select Delivery Schedule on the left to see all upcoming deliveries for your subscriptions. To skip a delivery per subscription, you can click Skip next to the delivery you would like to skip. Similarly, to unskip a delivery, you would click Unskip next to the order.