The answer is both win...but there is a necessary pre-condition to this happening
Consider this quote from The Master Key System to highlight the point:
Mental Power is creative power, it
gives you the ability to create for
yourself; it does not mean the ability
to take something away from someone
else. Nature never does things that
way. Nature makes two blades of grass
grow where one grew before, and Mind
Power enables men to do the same
thing.
There are no limits on what an individual can manifest if they manage to align their desire with their belief.
When there are two people involved and both appear to have conflicting desires then provided both are in alignment with their desires, the universe will blend the essence of those desires and satisfy both.
Referring to the example in the question about one wanting a child and the other not...there will be something about having a child that one wants (for example: the feeling of loving another, caring for someone, protectiveness, motherliness etc) and there will be something about not having a child that the other wants (for example: freedom, lack of responsibility etc).
If both are in alignment then circumstances will come about to satisfy and blend both feelings...perhaps a child that is extremely self-sufficient, for example. No-one can say exactly how the blended desires will be manifested - but they will be.
Now the main problem with all of this co-creating is that usually both are not in alignment with their desires. Usually one of the people wants something and the other person wants the other person to not have that thing. i.e. one person wants the child and the other does not want the child (instead of looking at their own real desire of wanting freedom, lack of responsibility etc)
In this case, the person in alignment receives the desire and the other doesn't and then thinks that the other person forced their wishes upon them (which isn't really the case). As I mentioned, if both are alignment, both must receive.
To make this principle clearer, let's consider how it applies in any competition where there are multiple people chasing one prize. Here things are slightly different in that there is only one possible winner of, say, the ultimate trophy.
In this case, there is clearly little room for the universe to create two winners because the manmade rules of the competition simply do not allow it. Here the one who wins is the one most in alignment with the desire of winning (the most "self-belief", or "expectation"), or failing that, the one with the least resistance to the goal of winning.