While camping, played two player, which was enjoyable. I got to be codemaster (which I love), and my partner got to be the guesser (which she loves). Some friends came by (who had not played the game), and instead of switching to a more traditional game, we kept the two player version going, with all the players on the same team. This was an enjoyable, casual way of playing, but something was lacking. We averaged 3 blue cards left each game (making sure to only play games in which red started), and yes, games where we had more left were a little bit more exciting, overall though the tension of traditional codenames was missing. So we made a tweak to scoring which fixed this.
With an average of three left, that means roughly 5 rounds in the game. and the likely fix is to have 3 rounds placing 2 blues, and 2 rounds place 1 blue, but that seems fiddly.
Instead we created a rule:
Anytime a round ends with correct answers, place TWO blue. Anytime a round ends from an incorrect (blue or neutral) answer, place ONE blue.
It is a subtle change to the two player rules, but did an immense amount to ramp up the tension and make the game feel more like the original. It also discouraged passing with just one correct answer, and encouraged guessing with limited information as neutrals were not as scary (and a little helpful). It also helped direct the codemaster to use 3 and 4 words clues.
It also turned it into a casual, social game, in which players could drop in and out midgame without impact (as all other players are guessers for the same team), making it a good way to train people for the traditional game.
2 player rules from book
Any number of players are guesser
Round advances as quoted rule above.