...is the complete psychological comfort.
My dual could have a completely different world view than me, in which case we would not be compatible. He could have totally opposite life goals, in which case he would be utterly unsuitable. We could be of different faiths, different political opinions, etc. etc. etc. and all these differences would combine to produce conflict after conflict.
So really, IMO, duality relationships don't really have any greater chance of long-term success in the real world than any other type relation.