I have a tempered glass screen protector on my 6+ and it's .33mm thick. When I use my Otterbox Defender it works fine inside the case. However, I always remove the plastic screen protectors from the Defenders when I use them, just don't like the feel and for me they always seem to decrease the screen's touch sensitivity, just my personal preference. If you want to try one without removing the plastic protector on the Defender, I would suggest you consider the Benks glass that is .15mm in thickness. One other thought though, by using both you may run into an issue with Touch ID. My guess is the glass screen protector under the plastic protector of the Defender may hold the membrane over and away from the Home button a bit.