you should always split your lifting and cardio up... weather its different days or 8 or so hours apart. If you do them together 9 times out of 10 you are going to be overworking your body putting it into a catabolic state and you will GAIN little muscle if any... most likely you will burn muscle away.

After 45-50 minutes of working out in 1 sitting your body stops burning fat and starts using muscle as its energy source.