How do I lose my "double butt?"

Squats is the first thing that jumps out at me that you can add to your routine to give you some results. It's really not all that hard to incorporate into leg day, and you get the added benefit of some other key muscle groups in the exercise as well.

Once you have that down, and assuming you really want to focus on your azz, I would also try throwing some other exercises into the mix to really isolate on your glutes. Some simple exercises that come to mind are flat bench leg raises, some wide stance barbell squats. barbell lunges, the thigh abductor machine (if one is available), butt lift bridges, and possibly some sumo deadlifts when you are up to it.

If you incorporate this into your leg routine, and assuming your diet is on point, you will have a smackable posterior in no time. Best of luck.