I have two cats that are six months old. They have never been away from each other. My boyfriend and I broke up and want to each take one. Will they have a hard time being alone for the first time ever?
I mean, at six months it will probably be easier than if they had been together for years. It really depends on how bonded they are. You and your ex may need to consider only one of you taking both cats. If you absolutely can't come to that sort of agreement, it will take time but they'll eventually get over it. Or you may each consider getting another compatible cat so that they won't be alone.
They will pine for each other and grieve. As lexiloo said, how bonded are they? If they are really a close pair I would encourage you not to separate if you can avoid it at all. Eventually they will move on and forget each other but it can be a slow and painful process.
My 6 month old boy just lost his sister and he is very depressed by it. He keeps crying and searching for her. But he will move on and so will yours. I do think cats should be in pairs though if that's what they are used to