From what I hear, BOTH Tamagotchi must have been in their respective adult stages (Tamas usually evolve into adults at 3 to 5 years of age) for 24 hours and both must have 4 friendship hearts when connected. Hope this helps!
Well, they cannot mostly....it is a well known glitch by the way, but they can if you connect the fe/male adult with the fe/male teen and have 4 faces, 2 hearts 2 faces, or just 4 hearts! this happened to me ONCE! it was...cool?