Does anyone here know how to use a second button in c64 games? For example, 1942 and 1943 have special attacks which require a second button. Obviously the c64 only supports 1 joystick button, but I read somewhere that the spacebar can be used as a second button for things like special attacks. However, I can't get that to actually work.
I'm using the vice emulator. The emulator definitely recognizes my keyboard input when in c64 basic mode (ie, no game loaded)(I can type stuff into the emulated c64 just as I could if it were an actual c64), and it recognizes it for emulator commands in-game (ie, I can pause by hitting alt+pause), but there's no apparent in-game response to the keyboard.

I'm afraid there is no generic answer here. As you remarked yourself, the standard Atari style joysticks as used on the C64 only know one single fire button. So if a game uses more than one, it is game-specific.

1942 uses the space bar to perform the rolling maneuver. 1943 uses Shift for the smart bombs. If those don't work for you in the emulator, my recommendation would be to confirm that these keys are not mapped to anything else in the emulator configuration (e.g. a used as a second joystick).

I was tinkering around after I posted last night and I found that 1942 uses joystick slot 2, while 1943 uses joystick 1 (srsly, wtf?). Upon further investigation, I found that the button on joy1 performs the loop on 1942 (though no keyboard key does). The reverse however does not work in 1943. I literally tried hitting every single key on the keyboard during the game & nothing does anything. I have also tried switching between the 2 english layouts available in the settings (symbol and something else), with no apparent change.

Have you tried turning the other joystick off?

The 2nd stick was originally off, I only turned it on when I couldn't get 1942 to work and read somewhere that it uses the 2nd joystick.