Another reason to carry a double adapter, if required you can ask to plug your device in beside theirs or you can be the good guy and offer a spare power point.
Mine has a surge protector just in case.
This is what I do as well. Also handy in some hotel rooms with few / inconveniently located outlets.
And I've never had anyone refuse to unplug their device long enough to add the adapter. Especially when they realize they can continue charging what was plugged in and something else as well. (Mine has 3 outlets, 2 USB ports, and surge protection. It is a little bulky but I always manage to find room for it.)