Simply print out the voucher between and present it upon entry to receive 2 adult tickets for the price of 1 during the Father's day weekend.
Took my lovely missus earlier this year for a romantic night out and we enjoyed it. It was actually relaxing but the view was only limited to lights after a certain point so you're obviously going to see alot more of Melbourne in detail if you attend during the day.
Usual conditions at the bottom of the voucher. Most important ones being that it is only valid between Friday 2/9/11 and Sunday 4/9/11 and entry does not include the "Edge Experience". That costs an additional $12 per person.
Hours and pricing here:
http://eurekaskydeck.com.au/pricing-and-hours.html
nice find!