If they are fairly evenly intermingled, it should be relatively easy to find out. If the two sets are separated, say the blood, heart, and skin are from one set, and the thyroid and lung are from another, it would be not so easy to find out, because normally you don't suspect that and only the blood is tested. But once you suspect chimerism, a thorough analysis should be able to reveal it. Although chimerism does not happen often naturally, it can be easily created with some animals (such as mice) in the laboratory. As a matter of fact, it is a routine technique in modern biological science.