You may have already done this part, but if you bought two tickets together, you'll need to transfer one to your buddy. (Go into your account [link at the top of this page], more specifically 'My e-Tickets', you'll see that you can transfer a ticket to someone.)
As for actually designating your buddy in the lottery process, that hasn't happened yet for this year's lottery. If I recall correctly, you do that as part of the lottery process, just before you make your lottery selections.