My wife and I have different mother tongues (neither is english).
We tried to bring up our daughter as trilingual - but so far our daughter is fluently bilingual - not tri. But its early days yet - she's just 2 and a half.
My wife and I speak to each other in English, which our daughter has picked up.
OShe has also picked up my wife's mother tongue. But not mine. The reason being, that compared to my wife, I hardly get to spend time at home with the kid.
Cheers....Newkid