Did you ever use the same coupon in one account twice by using two different cards?
I would strongly advise against this.....
The coupon is applied first, before checkout, so I don't think a different CC will help.
Also, just as does, -T- , I also recommend not doing it.
There are plenty of $1 to $4 coupons, so it easy to reg more then 100 domains a year.
I have been buying domains for development for long time but I recently started with domaining. I registered tons of domains using coupons at different registrars but had to dropped them all. Of all the domains I got with coupons, not only did I not get a single offer, I did not make enough money on parking, to even renew a single domain! This made me realize I was taking a wrong approach in selecting domains.
I don't know how many domains you have already bought with coupons but if it is a lot, then my suggestion is, if you are not already making a profit through the first 50 to 100 domains, then please reconsider your strategy, rather then investing more using coupons. You might get the domains cheap now but they will have to be renewed after a year!
After dropping over 100 domains in a year, I have slowed down on buying domains and started sharply focusing on few niche area's, so hopefully I won't have to drop the domains after a year.
I hope your domain selection is lot better then mine, so you won't have to learn the hard way after dropping domains, like me.
Hope my suggestion helps.