I've had it go both ways...sometimes it gives me the full overage and sometimes it just gives me the price. So, I usually stick to things that will use up the full $2.00 for my doubles and try to make as many of the other items in the order come out to zero.

For example:
15 blue bunnies - $13.20 -$15.00 in $1.00 coupons
2 clean team wipes - $2.98 -$3.00 in $3.00 off of 2
5 glade candles - $10.00 -$10.00 in $1.00 coupons doubled
2 snickers & 1 skittles bought with overage = $0.12 oop

The way I figure it you try to make your first $15.00 free or as close to free as possible and then your final $10.00 you look for items that are around that $2.00 mark that have $1.00 coupons. Some weeks are harder than others to make the deals work but this method has worked for me so far.