here it goes, first of all they have to be of opposite gender. I would advise you to connect them a lot when they were kids so they can become best friend, so it would be easier to have them fully love each other when they are adults.they will love each other at 4 yr old if you do that but they won't mate until 5 yr old.
what i did, in case they are two different versions, is I connected like crazy, didn't put them on pause, and once they were both 5 years old (or older) i connected and they had kids! hope this is helpful