Basically it's okay in terms of safety, but DMT really gives your heartrate and bloodpressure a jolt and on MDMA that could just be too much if you're having a lot of bad luck or if you have some sort of weakness you are not aware of. What one person can manage (or even lots), it's possible there are a few that can't. I don't know if it is going too far to say this is significant enough to keep in mind (because there are no statistics on this that I'm aware of), but rather that if this would turn sour physiologically speaking I bet it's because of this.
I have done it myself at least once with good result (wonderful in fact) but for what it's worth I found every other stimulant a bad bad idea to combine with DMT. I recommend you avoid. Really throws my breathing regularity off among other things.