You mean you want the Tamagotchis to be best friends - like in love?
They have to connect many times, and be opposite genders . If they are the same gender you can only get them up to 4 smileys, if they are the same you can make the friendship level 4 hearts. They have to both be adults to fall in love as well.
When your Tamagotchi is at any stage, you can use the cuckoo clock. The Tamagotchi wanders around under it, until it is on the hour (eg. 3:00, 4:00...). Then, the cuckoo comes and and makes the weirdest sound, and the clock disappears. The cuckoo clock can be used as many times as you like.