Kelly Tarlton's Sealife Aquarium is offering 2 for 1 general admission. Just show the voucher at the bottom of the Discovery Channel page when you buy a ticket. Tickets are $39 for an adult and $22 for a child. So get 2 adults for $39 and/or 2 children for $22.
From the T&Cs: "Two for One is only valid when purchasing full paying adult admission."
I don't think it is valid for child ticket, it is only for adult ticket.