auto loaded coupons are automatically loaded. If you remove it, it will be back automatically.
You should put both coupons on auto load. Actually, the auto loaded coupons are not reselected automatically when the cart changes. With next version of HikaShop, the auto load will automatically reselect the coupon if you change the cart so that it will switch between the 5% and the 10% automatically.
The system auto loads the best coupon possible. So if a group has two auto loaded coupon and one is better than the other, the most interesting one for the customer is selected.