Looks like they have an online form set up for existing subscribers to submit the request online. All you need to do is go to http://byo.com/2for1 , fill in your Zip Code & Account# (7-digit number on the 2nd line of the label. Mine had a 0 in front that had to be removed). From there, you confirm your mailing address, enter the mailing info for your "friend" and then chose a "Payment Method". I chose "Bill Me", which actually allowed both parties to be billed $16.50 separately!
Update: With no takers, I decided to gift my other subscription to a fellow brewer. While I got my initial subscription from paying half, I'd rather give it away than decline it. I know a guy who does a lot for other brewers, but doesn't receive BYO. Maybe some of you know worthy recipients too.
I stand corrected: BYO has informed me that the 2 for 1 deal is only for 1 year subscriptions/renewals. Sorry for raising > dashing anyone's hopes.
I have been contacted by someone here that was interested in sharing the 2 year subscription, and I hope he is still interested in the single year - it is still the best deal out there. If he is no longer interested I will repost.