Just for the arguements sake, here is a theroy one could propose against the method of taking them both together.
Ca2+ receptors is what regulate protein and creatine uptake into the cell membrane for metabolism. Insulin is the prime transporter into the receptors found in the cell membrane. It is not an ion per se (pa), Calcium regulates the transport of creatine and protein into the cell. The transport mechanism is limited, if larger amounts of pure protein are absorbed without other mechanisms being brought into play to improve absorption (insulin) it will be limited. Example;
Theoretical situation : If someone has a limited capacity for Calcium at 40 g. You consume 55 g of protein with 5 g of creatine. That gets you the final absorption of 36.7 g of protein and 3.3 g of creatine.