If you haven't responded to either invitation yet, he should get to choose which party he would rather attend. Explain that whichever party he chooses, that is the group he needs to stay with for all or nearly all of the party duration. Then you can gently explain the situation to the other Mom, and stop in with a small gift at some point if there is a natural break in the party he is attending, or right at the end. I doubt any reasonable person would have issue with this. If you have already committed to one party verbally, I would use it as a teaching opportunity that you can't now choose once you tell someone you are coming, you are making a committment, like a promise, they plan for you as a guest, and it isn't polite to skip out when a different invite comes along.