For the A/B test Date/Time method, you can only select one email. I would just have a batch smart campaign send out the emails, so you won't need the email program at all. Just make a clone of the email so there is one with each promo code and have one batch campaign send Email A at 7a and another separate batch campaign send Email B at 6p.
+1 on Devraj.
Also keep in mind that this isn't an AB test when you have more than one variable involved. Do Whole Email test at the same time OR different times of the day. Not both. It will be too difficult to see a real difference.