I'm sorry that you lost your kittens. It is hard to know what happened. Did these kittens ever recieve any vet care? Were these the first kittens you ever had?
It does sound like poison could be a strong possibility. I have to agree that I wouldn't get any more cats/kittens until you can find out what caused the death of these two. I'm sure you wouldn't want it to happen again. Was there any rat/mouse poison anywhere around?