Yes, it's very true that anemias can co-exist. And, worse yet, if you have a microcytic anemia (like iron deficiency) at the same time as you have a macrocytic anemia (like B12 or folate deficiency) it can actually cause blood tests to show a false normal results. I had never even heard of false normal test results until I was pretty much in that situation. It's crazy. (And it also sucks because you have all these anemia-like symptoms and your doctors look at your blood tests and try to tell you it's all in your mind when it really isn't...)
I realize now that this topic should probably have gone under the 'related disorders' section. Maybe a moderator could move it?