Do you have to order one at a time or can two be added to the cart at once?
Do I use the appleid associated with the joint iTunes account ? The iCloud/messenger appleid is different for one.
Practice ordering a 5S right now ... you will see the steps and be prepared for what to do tonight. Just don't click the final "buy" button
I'm pretty sure that you will be able to add both phones to your cart at once, although the danger is that if the Apple servers are overwhelmed, you could get 3/4 of the way through the process and have everything lost because of server issues. In years past, it has taken me a long time to get through all the hung servers.
Personally, I am planning on ordering them one at a time (mine first since I am more impatient than my wife
And you can buy the phones with whatever Apple ID you like ... you just need to enter the phone #, billing zip code, and last 4 of the social security # of the account owner for the phones you are buying.
To prepare your Apple ID, log in to the iOS Apple Store app, click on your account, and see if you have a credit card entered (and make sure it is the one you want to use).
p.s. In the past, using the Apple iOS app has worked better than trying to order on the computer. I don't know if that will be the case tonight or not.