No no problem. It's a 2 minute fire rating. If a fire started say in the garage 5/8ths one layer worth gives you 2 minutes to get out before your house catches fire.
Just a suggestion to use some kind of finishing screw. You could use 1" & 5/8" or 1"& 1/2" Let's not forget that as per building code that the ceiling in an attached garage has 5/8ths gypsum board along with any shared walls with the interior of the dwelling.just FYI. You want your fastener to...