Multiple characters were already possible in Bruce Lee (1984) on C64/Amstrad CPC and others. It had a 2 player option.
I wonder if someone finds an even older game.
Oh man, I remember Bruce Lee, loved that game BITD :) ... Hmmmmm, now you've got me thinking (I'm far from an expert on early computer games though). I think the question will largely boil down to what the definition is of "multiple protagonists".
I believe the OP meant games where one single player can control multiple protagonists, not two player games where the different players have different characters (in which case there is Spacewar!
I recall [url=https://en.wikipedia.org/wiki/Combat_(1977_video_game)]Combat[/url] for the Atari 2600 (1977) where you can control different vehicles, but think those would qualify as separate games rather than multiple protagonists.
A bit of research pulled up (1971), a text game where you can control different players on your team. There is also [url=https://en.wikipedia.org/wiki/Dungeon_(video_game)]Dungeon
(1975) where you control each character in your party separately. (I'm unfamiliar with these two games personally so I'm not sure how far the "multiple protagonists" definition applies).
This is getting rather interesting, I wonder if I should repost it on the main/general forum?