Did you start both your tamas off as babies at exactly the same time?
If you started one tama off in the morning, and the other tama a few hours later on the same day, then the 2nd tama will evolve a few hours later.
As mentioned before, pausing a tama slows down the growth process, but generally, this is the way evolution goes (based on an UNPAUSED tama):
Baby = 1hr, then evolves into Child/Toddler
Child/Toddler = 24hrs, then evolves into Teen
Teen = 48hrs (V4) or 72hrs (V1-V3), then evolves into Adult
V4 Adult 24hrs, then goes job hunting
Lots of ppl make the mistake of asking how old (how many years) it has to be before evolving.
Actually it is a matter of how many hours it has been running - not how many years old it is.
Best thing is to try to remember what the time was (and what day it was) - approximately - when you started the tama - this will always give you the best guide to the times your tama will evolve