In the past, I (and others) have stated that you can use both a manufacturer's coupon and one of the e-coupons together for the same product. However, Andrea at Mommy Snacks has brought it to our attention that using 2 coupons for 1 item is not really what the manufacturer's intended. Click here to read more information about it.
Like her, I plan on using the coupon that brings me the most benefit (which is usually the manufacturer's coupon since it will double and the e-coupon will not. I will also no longer state that you can use both coupons (although I will probably list both so that you can choose which one to use yourself).