Even if you are subscribed to both subscriptions, we track them individualy. So, if you go back to a single subscription, you will return to the rank relative to its streak.
For example: if you are at the Reality Bender (Level 3 Dual rank) for having a Fantasy subscription for 7 months and Sci-Fi for 1, you will go to Fierce Warrior (Level 3 Fantasy rank) if you cancel your Sci-Fi subscription, or to UFO Research (Level 1 Sci-Fi rank) if you cancel your Fantasy subscription.
To check the tier list of all ranks, access this page.