That's a really tough one and I would be fuming too.
I had a similar, (although not possibly not quite as bad) predicament a few months back where my fiancé (who I've been with 4 years) was not invited to my cousin's wedding - although in fairness the cousin in question had not met him (but then again my other cousin's girlfriend of 1 and a half years who they don't know that well either was invited!!) when I discreetly mentioned it to another relative, my cousin rang me all apologetic saying they didn't have the space but she said I could bring my fiancé to the evening do, but that just wasn't feasible as we live about 6 hours from the wedding venue, so I wasn't exactly going to ask him to do a 12-hour round trip only to spend most of the day sat in a hotel room on his own, in a place he doesn't know, then walk into a party full of drunk people, on his own. In the end another guest dropped out at the last minute so my fiancé was allowed to come, so that sort of solved the issue. But if that hadn't happened then I was going to go on my own without him (which I was *really* embarrassed about since his family have been so welcoming to me and invited me to all their family events, even when they hadn't met me!)
At first I had toyed with the idea of just not going at all out of principle, but then I thought deep down I don't really want to cause a scene on their day and make it about me, and also I would quite like them and the rest of their families to come to my wedding too, so there's that to think about... Also when it comes to these things, it's almost always because they've got very limited space and have had to make some tough decisions, so sometimes it helps to be understanding.
Then again, from what you've said about how close you see them as, I would be really hurt by it. Is it too late to retract the rejection of your other friend's wedding? Personally that would be my first choice - go to the other wedding! (And maybe have your fiancé still go to the first wedding, if he wants to see his other friends... ). It's a perfect excuse for not going without causing a scene or risking looking petty, and you'd be having a good time rather than moping around all day while everyone else has fun.
If that's not an option... you mention "waiting around near the venue" – does that mean you'd be travelling a long way? if that is the case maybe your fiancé could discreetly say, "look, it's just not practical for us to come to separately..." (if that's the truth of course) and see if they could squeeze you in as well... For our wedding, we've just said that anyone who is travelling a long way is invited to the whole thing, regardless of how close to us they are, as I just think it's totally unfair to expect people to travel a long way and then only let them in for a couple of hours when the best food and free booze has all gone!!
As for how to invite them to yours... tempting though it would be to get them back by only inviting them to the evening do, I think you risk coming across as petty if you do that, and I think taking the moral high ground would be more dignified. Hopefully when they receive your invitation with both of them on it it will make them think twice about how they treated you and maybe they'll be a bit embarrassed, but your friendship will be intact.