How do you know when to double coupons?

I have coupons and I read the really small fine print saying the coupon is good for one purchase can you still double coupon i have no idea how that works.How do you know when to double coupons?
A store doubles a coupon up to a certain amount. my store is up to $1.00. If the coupon says I get $1.00 off an item, I get $2.00 off an item. Most stores that double coupons only do it up to .50, so when double you would get $1.00 off!!