My question is: how mutually exclusive are these quests?
When I join one will the other immediately become unavailable? Or can I join both and only at a certain point I have to make the final decision? If such a 'point of no return' quest exists where you're stuck with your choice I'd like to know about it (go easy on the spoilers
). I would like to make an informed decision ingame about which side to join and in order to do so I kinda have to get a taste of both sides first...Also, what about the Battleborn/Greymanes in Whiterun for example. Will helping one (free some guy out of Thalmor prison) prevent me from doing quests for the opposite side as well?
Any further information about mutually exclusive questlines is appreciated.

