I wrote them and received this reply:
"Thank you for contacting CVS/Pharmacy ExtraCare in regards to our BOGO sales. Should CVS run a BOGO ad for an item and the manufacturer is also running a BOGO coupon for the same item, the coupon can be accepted provided it is a legal MFG sponsored coupon and the item and purchase meets the criteria stipulated on the coupon. What this would mean is if the manufacturer is running a BOGO promotion and CVS is running one as well and you have (2) coupons for the promotion, you would receive the item for free."
It still makes it sound like I need two B1G1 coupons...