It would be nice since you cannot use the same COUPON CODE in two separate promos, if the system actually prevented you from submitting a new promo with an existing coupon code. As it, all feels great until someone tries to use it. When two promos exist with the same coupon code, neither one works.
Or, better yet, make multiple promos allow the same coupon code again.
Example: Buy 1+ you get x. Buy 2+ you get x and y. Type code gimmexANDy! during checkout!
When you type it, they are both looked at for their criteria, and if coupled, both can be applied.