They can only mate when they are both in their adult stage(usually 3-4 days old) and must be opposite genders and are in love (check this in the friends list to see the 4 hearts) and then they will fall in love, you will see the fireworks and they will have 2 babies, one on both screens.
If your tama does not have a suitable mate, it is visited by the alligator looking person( usually about 6-7 days old) and they find a mate for you, they will ask if it is true love and if you say yes they will mate and have 2 babies.
To get your tamas to connect go to the fifth icon on the screen by pushing the "A" button until it is highlighted. Push the "B" button when it is highlighted and put the tama's witin close distance and press the "B" button until it says "Connect" They will play games with eachother and automaticly begin to like eachother. After about 10 times, they will fall in love.