Since there are many neurological diseases besides botulism, some viral, I'd isolate the two sick ones and do a general coop/water/feeder clean up. After that supportive care is all you can do. Vitamin deficiencies can also cause neurological symptoms, but again, it sounds like you are providing a varied diet. The only time I had a deficiency was when I relied on a cheap commercial chick feed and had slacked off feeding greens and things like egg yolk with the fast growing turkey poults. Their problems quickly resolved when I dumped the feed, switched brands, and added mealworms and egg yolk. If you have any more chickens getting sick you might want to get them tested. I'm pretty sure some neurological diseases are contagious, not sure if any are zootonic so be careful.