I did just this last week. The ATT rep said that for three hours both of my phones would be in "duel mode." Both phones would recieve the same calls/texts/emails etc.
10 minutes after I walked out of the store, my Tmo phone said "no service" & my iphone was getting all my emails/texts/etc. The ATT guy forgot to give my line a data plan (HOW?!) so I didn't get that until I actually called about an hour later. (My SO ported his number from Verizon the same day & his had data services 45 minutes after we left ATT).
Hope that helps!