You can wait until you get to the box office shortly before your showtime as long as your viewing is not sold out. It would be safest to wait until you get there to obtain both tickets. Just get there a little bit early and you should be fine. You can check how full the theater is getting for each showtime on the Alamo Draft house app.
The best way is to go to the box office to get the free teacher ticket
My mother (a teacher) was told by an employee that you can purchase the ticket online and then be refunded the ticket price when you go to the theatre and show your school ID.
You may want to call up there & ask, when purchasing tickets online many times you will pay extra fees through different companies so it ends up being more money than if you were to pay for your ticket in person & they will only reimburse your ticket cost, not anything extra for ordering online.
I can't speak on those tickets specifically, but every time I've had a free ticket (birthday reward from their rewards program, given tickets by the theater for a promotion similar to the one in question), I've had to redeem it at the time of purchase AT the theater. I've never been able to do it online.
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