I have 2 lines too.
So I tried the first one reserved to pick up at the store. When I tried the second line, I found out that each Apple ID could only hold one phone. So I decided to ship the second phone to me, that worked (check out with guest account). Then, I did the first phone again to ship to me and canceled the reservation at the Apple store.
Now I have 2 phones shipped to me at 24th.
Well, the apple store kept giving me errors when doing those 2 lines (all happened when checking with ATT's database), I had to try over and over again to get them through.
Talked to a Apple guy this morning and ordered over the phone. He told because of the way AT$T wants to handle the ordering, there can be only one iPhone per order, so he put two orders in, for one iPhone each. Same Apple account, same credit card.
(Existing Family plan with two lines, both iPhones)