It can be diffuclt to know what plans are when you're asked to make decision months in advance.
So they missed their chance to attend the dinner party. That's what happens when you don't accept an invitation when it is extended, just in case something better comes along. Because really, why else wouldn't someone accept an invitation immediately? Either the date is free or it isn't. They gambled and lost.
Sometimes you do have to check on a date--you might not remember the exact date of an event, or you might be trying to make plans with someone else and have two or three dates that semi-reserved because you haven't figured out when you are meeting. Or, over the holidays especially, you might know you have a family get-together that you want to go to, but the date hasn't been finalized yet.
As an example, a very close friend of mine was born on New Years Day. Some years she throws a New Years Eve/birthday party. Some years she has a small party on New Years Day. And some years, she doesn't have any sort of a party. We're close enough that I would chose to attend her New Years party, whichever day it was, if she was having one. But I wouldn't know two months ahead of time when it was going to be, necessarily. So if I got another invitation for New Years, I'd have to check with my friend to see if she was having a party.
But if that's the case, then, unlike the couple in the OP, you tell the person extending a new invitation that. "Oh, we'd love to come, but we need to check with Dave's family/my brother/the Simpsons first, as we've promised to go to their big holiday party. But they hadn't set the date the last time we talked to them. Can you give us two days to see if they've pinned down a date yet?"
Especially if your host has told you there is limited seating and that she'll take your inability to answer as a "no." If you really wanted to attend that party, wouldn't you ask for a day or two to check out what the other plans are, and then get back to your host? Most hosts, two months out, would be willing to wait a day or two for a response, I think. But probably not a month, as they might want, as the OP did, to invite someone else if there's space.