I think it depends upon the type of party and the hosts' expectations.
Many Super Bowl parties are drop-in-and-out kinds of things. But not all are. If you are one of four people, that's very different from one of 25. The hosts of a smaller group might not care, but some might.
Some days, e.g. New Year's Eve and Halloween, are times when people are likely to be invited to more than one party, and therefore hosts often make them open-house types of events, because they don't want to make their guests have to choose. In such cases, hosts don't seem to mind getting a 1/2-acceptance -- (although I always try somehow to make all the hosts feel that I'd really rather be at THEIR party all evening, but I "have to" drop in elsewhere, too.) Two of our friends both always have Halloween parties the Saturday night before (or of) Halloween. They both know it and understand that their many mutual friends will go to both (and one couple even sometimes comes to the end of the other party). Unless it's something like a restaurant reservation or a dinner party, I think that hosts would be kind of unrealistic to expect that people won't be going to other parties besides theirs on those two nights especially, and also often Fourth of July and high school graduation season, when, in our community, lots of grads' families have casual backyard open house parties for the whole class and their parents, so there are usually more than one each evening, and everyone just sort of makes the circuit.
Super Bowl Sunday seems to me to be kind of on the line. I wouldn't mind getting a 1/2 acceptance. But your friends did mind, right? And I can't say they were unreasonable. Because although the official invitation didn't come until after you received and accepted another invitation, it sounds, if I am understanding correctly, that the time, date, and the fact that you would be invited were already pretty clear. So from their point of view, it might well feel like you had plans with them, and then something you preferred came up, so you're ditching them for half their party.
The point is, it could go either way, and the important factor here isn't which rule applies -- it's how they feel about it. That you cannot control, nor is it subject to rules about Super Bowl parties.
It still doesn't mean that you can't go to both parties. But you might want to think about how you communicate it to them. Don't be cavalier or make them feel like they are wrong if they are hurt. Don't say self-justifying things like "Well, Super Bowl parties are usually open house style" or "You don't mind, do you?" Let THEM be the gracious ones and tell you it's fine if they come for 1/2 their party -- don't YOU tell THEM. If they are hurt or offended, don't try to argue that you were right, just apologize for the misunderstanding and don't do it again.
