I may be completely wrong about this, but off the top of my head I cannot think of such a law in Scripture (eating milk and meat at the same time) and am inclined to think it is a case of where man's law has been added to God's law despite the prohibition against doing so. I will have to do some research on that when I have more time. The closest thing I can think of in the Law is that they were not to boil the meat of the lamb in the mother's milk.