dont take my word as gospel because i dont want to be responsible if you get blown away but in all my experiences with car batteries, boat batteries, satellite communications, air conditioning units, video game repair, ive seen multiple instances of multiple things grounded on either a negative or multiple seperate ground screws that all connected to the same damn piece of metal.
now with that being said... does the rule change depending on AC versus DC? I dont know. i guess car and boat batteries run on DC and it seemed fine to stack your negatives. and household power runs on AC and if you look at your air conditioning condenser, youll have 1 ground going to a screw that just bolts to the metal. i dont know why it would be dangerous to add another ground wire to same screw versus drilling a seperate screw somewhere else
so im 99.9% sure youd be fine but im not an electrical engineer so...