Can I just say, it's really not as simple as brightly saying 'rehome them!'
I got my elderly cat from a shelter eight years ago now. I wanted an adult cat because as sweet as kittens are, I'd just moved into a new house and wanted to keep it 'nice'.
My cat was just lovely. Very friendly without being overly demanding, happy to lie by the radiator in winter and sprawled out in the garden in summer, fully litter trained, very patient with various small children 'loving' him - I don't know how he came to be in the shelter but I think he may have been with a young family.
Anyway, he had been there eighteen months
My cat loved people. Loved being my my side and loved sitting on friends laps. He must have been so lonely in that shelter, even though the staff were very kind.
He was PTS sixteen months ago due to cancer and I'm so glad he could live out the rest of his days happily with me. But there just isn't a big queue of people like me.
I was going to go back and get another adult cat from the shelter but life had other ideas and I found two black cats in an abandoned house - they'd just been left
I wasn't really looking for another cat at that point, but what could I do? Because of their history they aren't as nice as my other one, they are quite nervous and although one of them is sometimes extremely affectionate and loving at other times he acts like I'm a cat serial killer 
The point is, rescue centres are going to be full to bursting soon with kittens. I'm afraid people do go for them and you can see why - they are gorgeous, playful and sweet and they are plentiful. Meanwhile, loving cats like mine get ignored. Trust me on this.
Even if you get the odd person looking for an adult, they take one. One cat - and how many others do you think end up waiting? I can also tell you that a responsible rescue centre will definitely keep them together but that means their wait will be longer.
I fully appreciate there are times in life when circumstances change and I wouldn't tell anybody they should never ever rehome a pet because life happens. However, I'm concerned by the blasé answers on here which suggest the cats will be instantly snatched up by loving owners. Loving people who like cats tend to already have them. It's a rare moment when a loving and cat free home walk into the rescue centre wanting two adults 