Good luck with that one. Unless they are the little useless ones worn by the Harley people you will only get one in there, especially if it is a full face. When we are 2 up we put one in the trunk and carry the other one. David
The RT's Frunk will hold 2 full-face helmets... kind of! We use modular helmets because they work better for eyeglass wearing fools like us! o If you leave the fronts of them open you can manipulate them into the Frunk and get 2 to fit...
Otherwise it's half-helmets and limited protection!
There's an outfit that makes a helmet lock set that fits on the handlebar ends... I'll try to find a link for them...(Kewlmetal perhaps??)