I've had at least 2 cats for the past 20 years - and mostly, 4 or 5 at a time. Multi-cat households are possible - but they require a lot of dedication and effort (e.g. with careful introductions and ensuring they all get attention), a lot of patience, care with feeding so more dominant animals don't get all the food, the "right" house (as in plenty of spaces for individual cats to "escape" to and feel safe in, as well as ready exits so they don't feel "trapped"), the "right" neighbourhood so cats feel happy to go out when they need to get away from the others, commitment to lifelong care and vet's bills .... and of course, the "right" cat(s) because not all of them are capable of living with other cats, however dopey they are with humans, and on this, you must be guided both by the observations and recommendations of the shelter as well as your own gut instinct.
But overriding all those considerations, and this applies to any pet - whether it's one or twenty (not that I'm advocating 20 cats) is that ALL the family have to be on board 101%. No reputable shelter would let you take a cat if they knew your DH had reservations - however unreasonable you think he is. And if you get a cat from elsewhere - potentially for example, an advert out of a local paper, you'd not only possibly be encouraging the twats who breed for profit and/or who can't be bothered to get their poor cat(s) neutered, but you'd also be storing up a huge heap of trouble in your relationship.
I do see why you think it's unfair when most of you want another cat and he doesn't. But really, it's the adults whose opinions count here and this is a similar(ish) dilemma to the old chestnut where one person wants a baby and the other doesn't because there isn't a compromise. It's either one thing or another.
From the cat's perspective, and that's both of your cats I mean, it wouldn't be fair to instigate this when there'll almost certainly be tension in the house as a result. Cats are extremely susceptible to stress and their getting to know each other won't be helped if they sense an atmosphere. Worse still, what if your DH "takes against" the new cat .... as in ignoring it, pushing it away, or (and obviously I don't know him) taking out his frustrations at you on the animal. I know that sounds extreme - but it does happen, and it's not fair to potentially expose an "unwanted" (by DH) cat to any of that.
The only way you should get another cat is if DH buys in. So you have to talk to him - sensibly and objectively. And for his part, he should at the very least be prepared to properly explain his objections to you rather than saying "I've said no so tough". If he truly fears the existing cat would kill the new one, you need to talk it through sensibly. As others have said it's very very rare for one cat to kill another. Unneutered toms are the most likely to have terrible fights - but I currently have 4 neutered toms (plus a girl) and they all get on fine, they really do. Has your existing cat ever shown any aggression to neighbouring cats for example ? .... or does your DH have past experience of owning a cat who got into bad fights ? Or, is the "killing" thing the first thing he could think of to block the idea ? ...... does he have other concerns really, like the additional cost and responsibility, the extra work. If so, he owes you an honest explanation.
But at the end of the day, there's no law which says you have to have x number of cats, even if you do have one already. Agree that for their sake you shouldn't raise the kids' hopes while their dad says no.