Yes I'm with you too Sharina. If there were a group of us that did everything together, I would be miffed to "find out" that one couple had been invited to spend the weekend with the other couple, and not being told about it. However IF the couple that did the inviting said something like "Sharina, we've invited so and so for the weekend, of course we'd love for you to join us, but we just don't have the room I'm afraid. As we would like your company as well for a weekend, then we will arrange something with you too very soon.
Then i'd be absolutely fine with that and I'd not mind at all. To do this so secretively puts red flags up for me, as this type of thing has happened to me in the past, which is why I chose to pull away from the friendships. I'm afraid I don't have a lot of trust in people. I'm an honest person, and had that been me, I'd have told the second couple straight away, and I'd be arranging a weekend with them also. Surely that is what true friends do? Not secretively arrange something behind the others back?