Adding a Subscription vs. "Move to Subscription"?

I'm hoping to provide multiple types of subscriptions on my site, based on different access levels.

The one subscription is finite ("Subscription A") and gives access to posts protected by shortcode # 1.

The other subscription is indefinite ("Subscription B") and gives access to posts protected by shortcode # 2.

I would like users to be able to subscribe to either or both Subscription A and B. But it seems that in Membership this is not possible. Users that are part of Subscription A have Subscription B presented on their account page as a "move to" option. This seems to mean that when they "move to" Subscription B, they loose Subscription A. But I don't want them to loose anything, I just want them to be able to ADD a subscription to their existing subscription.

I realize that from the way my question is phrased it might seem the answer is to add a Subscription C that is a combination of A and B. But I cannot see how, for A is finite, whereas B is indefinite.

And in case it matters in devising a solution: "Subscription A" gives access to posts in the blog index that are set to expire after a certain period (currently this expiration is set with another plugin). The idea for "Subscription B" is to give access to different posts that are not set to expire.