Xrandr, nor any other program, is detecting my second monitor; as in, xrandr acts like the second monitor doesn't even exist when it's connected. I think it could be due to the fact that both the monitor and the graphics card that the monitor is connected to are fairly old, and uses a VGA adapter. The light on it goes on briefly, then it just goes to sleep.
To further complicate things, I can only seem to get one monitor to work at a time, and it only seems to decide when I boot up the computer. Once the computer is running and I try to switch monitors (as just connecting the other one doesn't do anything either), I either can't get it to work on the old, second monitor, or it'll work on my primary monitor but with the resolution messed up. I've never encountered this problem running Windows in the past.
My OS is Devuan, and my main graphics card, the one that's being used, is an AMD Radeon HD 6450. I am not entirely sure what graphics card the old one is, and can't seem to find it anywhere on my system, except that I know it is Intel.
While I'm not sure if this will be any help, xrandr outputs 3 screens: HDMI-1 is the one that I'm using. I also see a "DVI-0" and "VGA-1", though both of these are disconnected. Though it's worth noting that DVI-0 and VGA-1 appear whether the second monitor is connected or not, and HDMI-1 is always whichever monitor that I'm currently using.