I don't have exact weights, but according to the vet, one weighs a bit too much (he's chunky) and the other weighs not nearly enough (he's bony). But, the skinny one has put on a few ounces with us trying to feed him constantly, so we're happy about that. (He's very old, and seemed to lose interest in food for awhile there.)
I think, in general, you know your cat is at a healthy weight if you can run your hand over their back and it feels smooth, not all spine. It's especially bad if you can slide you hand most of the way under the spine - that's a very underweight cat. Then, if the back is good, try picking them up. Can you feel a defined waist, and a bit of rib (but with flesh over it)? That's good. But, if it feels like your cat's got a solidly round abdomen - ball-shaped - and no waist, then they're probably overweight.
But, of course, the vet's always the best judge.