What Janet said.. Totally fine if you do it correctly - martingale on the snaffle rein, with stops, and to be sure it's not interfering with the curb rein the curb rein will go on the outside of the martingale leads.
I used to do the following to make sure I got it right. Bridle the horse, and put only the snaffle rein over his neck. Attach martingale. Then put the curb rein over his neck. Once it's on and attached, you can take either or both reins back off his neck if you need to lead him and they won't get mixed up when you put them back on to ride.
Great, thanks! I've got a horse who jumps fabulously in both a standing martingale and a running, but I vastly prefer the running for safety issues. Just wanted to know it wouldn't have any ill effects.