I know how....I keep asking that question also and now I know, you just keep on doing presents and gift, using Love Potion won't help anymore when your friends has 4 hearts ( I tried to use 33 Love Potions on each, but nothing happened)....When you choose Present or Visit and you saw the Two Tamagotchi are looking left and right, that means that they are gonna marry each other, I think they both are going up and fireworks are gonna show up, you'll see the female tamagotchi making two babies, she speaks with the two, and then one of them will go to the other Tamagotchi, the father.
I think Bandai did this so if two Tamagotchi, male and female, want to give presents and don't want to marry, because if the marrying process is fast, you might accidentically marry another Tamagotchi that you didn't want to marry.