I think because they had both had too much excitement (poppers, beer, cross dressing, thrifting, attempted fraud) for one day. Brains frazzled

I have no idea why that thought has never crossed my mind. Like you say, Mac's other had was free and fully functioning here. Tnere was no logical reason for him to not hold the poppers to his nose himself. I guess it was just Charlie holding it so that the situation comes off more silly to the audience.