The trick is to get the coupon in HTML format in your email.
Once you've got one from Home Depot and one from Lowes print multiple copies of each coupon. Use the Home Depot coupon at Lowes and the Lowes coupoon at Home Depot. Repeat. They try to scan in the bar code at the bottom but it doesn't register on their system. It usually takes a manager override to get the discount, but I've never had problems with it. I've used it for $350 off our $3,500 flooring purchase and I've used it to get an additional $10 off a $100 tool purchase.