Question is pretty much in the title: is it possible for an alliance to win as one, instead of a single player being the winner?
To be honest, I doubt it, since the same question on Endless Space had the following answer:
the victory conditions imply that you cannot by default have an allied victory …
But while Endless Legend is similar, it is another game, so maybe something changed?
Since version 1.5.0, Pact of the Seas, it’s possible to enable “Shared Victory” at game creation. This will share the victory of an alliance member with the rest of the alliance. From the original patch notes:
Ever had a multiplayer game where the last dozen turns are rife with suspicion and mistrust, as you and your allies all separately angle for the victory, knowing full well that only one of you would win, hoping there was a way you could share credit for your hard-won victory? Hope no more! We added an option at game creation that enables shared victories: basically, if you win, you win for whatever victory you earned, and your allies get the shared victory.
Make sure you tick the option at game creation, because it can’t be changed later on. Take comfort in the knowledge that if the option is activated, you are safe(r) from last-minute betrayal!