It can be a rather tough process, but if you so feel the need both Tamagotchi must have four hearts under eachother when you select the notebook icon and look under friends. The Love Potion (honey) code will give you a little bottle that will automadically bring the friend levels up to the very top. As simple as this may sound, there are a few catches. They must be the same age (or extreemly close to it) and of the same version. Age 6 or 7 (and if your lucky, sometimes in the beginning of age 8) will be the mose effectave in the production of the twins. If you run out of time, I suggust you use the matchmaker quickly, as the new version four Tamas will also have to have near the same personality.
Bandi just keep getting more realistic with these little guys!
Hope I've helped!
-Tama-Nut