Nope, it is not true about a twins code.
On the V1 and V2 Connections/Connexions the only time you get twins is when the female gives birth to two babies of the same sex and one immediately leaves the screen to join the male.
There is no 'cheat' or 'code' to get twins on IR ConneXions / ConneCTions.
This is something that has also been confirmed by BanDai.
We had a member claim to have gotten twins with a ROM test during this Christmas holiday period - but they didn't know how / couldn't explain how they got twins and have never posted any replies to back up their claim since.
It doesn't matter which one you have heard, nor if it is one that sounds like it could work - I promise you that it is a rumour that surfaces on a tediously regular basis - appprox once every 3 weeks! (...and it is not true... ) - so many people have tried codes and cheats - and not one of them has worked.
Sorry!
Best, etc.
TamaMum