You can use couponmom's database to find out which insert that BOGO coupon comes from. I cut mine out a while ago so I don't remember.
lucyrevenge gave a perfect explanation of how to do the deal. I've gotten 6 packs of those razors this week.
One word of caution, because I made this screw-up the first time: if you are getting the women's razors, please make sure they are the ones with the pink handles, not the purple. The purple ones rang up at regular price, not the sale price, and that took about 15 minutes to fix. Now they MIGHT ring up at $5.99 for you, but if they don't, that will be why. When I went back again, I decided to take no chances. Pink it was.