We are doing it and we are doing it well I think. The girls appear to like it and know where they stand. They (the little one more) sometimes say 'But dad lets me do x' or 'I wish we all still lived together' but you basically hold your own line for the former (which is far easier to do consistently when you are separated than when you are with someone who undermines you - RESULT!!!) or discuss sensitively (the latter).
We do 4 days with me 3 days with him and we live nearby and we do all their birthdays, xmas, special occasions, parents eves, concerts etc together because we are their parents and that's what parents do. We have a very open house policy if his gf is around when he picks up they both come for coffee. If my bf is around it's the same.
However, if he was being a dickhead about something as simple as one event like yours I would not flog myself to be around him and change arrangements to suit him... Open the invite to special occasions, but make your own arrangements - there's no need to travel together and spend over and above the necessary amount of time together if he's doing your head in. These are tweaks that you make over time as you get to learn how much of him you can deal with, and then you get zen like about it. Exh will change arrangements so that he loses significant time with the girls, so he can attend sporting events, then moan to others about how little he sees them!!!! It isn't my responsibility to fix that, so for them, I just make sure that when they're with me I don't put other shit first, but I don't ruin my own plans to accommodate his hobbies.
There are always gripes and games and things that piss me off but I basically vent at others (not the dds) about that and raise issues/step in only if I think they are serious e.g. dd1, 14, having a party at his house and he had put no limits on how many kids invited, intended to go out and leave them to it, and thought they were not going to bring alcohol.... Pahahahahah!!
The point is about making sure you operate on broadly shared principles of what's best for them but that all that spadework is done backstage not in front of them, even making some rules explicit so less resentment builds, like 'you do spellings and english homework I'll do timestables and maths homework'. That simple, and out loud. Also maintaining open enough, regular communication about the children that you can raise problems without it being really awkward. So e.g. we talk often about how is dd2 getting on at after school club because she hates it some days and we need to work on an alternative plan. If we weren't checking in about it regularly it would be really difficult to discuss the background to it when trying to arrange something else. (Not that he would arrange it in a million years - but there's an example of gripe I don't rave at him about I just make sure he is part of the conversation and the decision).
I miss them horribly and it still feels weird when the dds are not there but it's only been two years and I have lots of nice times doing other stuff I never used to do when I was married, like see people, go away for weekends, and have a normal adult relationship with my bf with lots of sex and both my girls are really happy to see us both happy with other people which is really nice.