Using Dual Sim Phone

[AC Question]

I am picking up an unlocked phone originally for use on T-Mobile with a prepaid sim for instances I might not have coverage; however, I have began thinking that since I currently have one of those $30 T-Mobile Sim cards that I was using to test the network that maybe I should keep that and primarily use it for back up data and instead port my # to AT&T...

Do any of you all have experience with doing this type of thing and how do you do it? My phone is a dual sim standby so Im not entirely sure what that entails and how it all works.

Thanks for the help