The answer to this question is Yes!
SiriusXM offers what is called a "Family Discount" that starts at a base price of $10.99 per month if you have an active, full-price plan already on your account.
To qualify for and maintain a reduced subscription rate for additional radios on your account, at least one radio on your account must be subscribed to an active, full priced subscription to a qualifying plan and package.
If you have more than one SiriusXM account, combining all your subscriptions onto a single account could not only save you money, but would make managing all your subscriptions more convenient. By combining multiple radios into one account, you'll be more likely to qualify for a Family Discount.
Changing the number of radios on your account or type of plan/package the radio is on could impact future pricing of your subscription(s).