For help on this you should post it in 'Ranch Talk' where more people
will see it.
Have you talked with your vet? Have him do an autopsy if another one dies~and the sooner you find out, the better. I would say it could have to do with the milk, but with it happening with two different cows that makes it highly unlikely, but not impossible. Have you checked their bags? What is their milk like?
Good luck. Sorry this is happening.