that would've been modded or using an server plugin or something as its not possible in pure vanilla
Not since 1.9; boats can have two riders (the first player to get in is the one who controls it):
Boats can support two riders, including mobs. Mobs can't exit a boat and are trapped until the boat gets destroyed, or until the player uses a fishing rod or lead to remove the mob. This can be utilized to safely drown mobs.
I saw two players in one boat in a yt video (it was vanilla minecraft)
Not since 1.9; boats can have two riders (the first player to get in is the one who controls it):
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?