If these are young calves - you need to seperate them. They are acting on their desire to nurse on their momma. When she ain't there, they turn to whatever is. This can lead to the navel opening up from all of the sucking which then leads to infection which can lead to serious trouble. We have this problem frequently with our holstein bottle calves. Getting them to drinking from a bucket instead of the bottle helps a lot. Then they need time to get the nursing desire broken.