Please do yourself a favor and get them checked out by a vet. I realize that they were probably dumped and that there's a good chance that there's nothing wrong with them, however a similar act of charity on my part resulted in the deaths of 3 of my 4 cats about a year and a half ago. We still don't know what disease the young one brought to our cats, but we strongly suspect that it was feline leukemia - my cats were vaccinated, but they were middle aged. The sad thing is that the young ones often don't show signs of it, even though its absolutely deadly for older cats. Unfortunately if there is anything wrong with them, its probably too late for the one you already have if they've been allowed to mix, but I would still run them to the vet, just so you know that they're disease free. While you're there, though it costs a few bucks, start them on their vaccinations.