I'm not confident I see the problem well enough, but I understand that if you make matching soffits on both sides of the room ten the soffits will combine to eat up nearly half of the ceiling.
What I expect is that there are two ducts running down one side of the room. Could one be rerouted to fit in the soffit on the other side?
What I expect is that there are two ducts running down one side of the room. Could one be rerouted to fit in the soffit on the other side?