Yes. You can start subscribing to SATs any time after you begin reading Discourses, so many students hold both subscriptions at once. If you are a Discourse subscriber, the Discourses will be your Primary subscription that makes you an Active MSIA student. A SAT subscription is optional while you have a Discourse subscription.
Once you complete all 12 years of Discourses, a SAT subscription becomes your Primary subscription and is required to remain an Active MSIA student.