Originally Posted by
irishguy28
No. Second seats (whether for the person themselves, or for their viola, etc) do not earn miles in any programme. Any attempts to achieve this outcome would breach the terms of most programmes, and you run the risk of having your account audited and closed.
This article disagrees with that - for some airlines including UA.
United Airlines says it double-credits miles to a traveler's account for the purchase of a second seat, whether for a cello or because the customer needs the extra space.
I believe it's RDM only.