clang -framework CoreGraphics ./AlignTwoDisplaysHorizontally.c

AlignTwoDisplaysHorizontally.c

#include <CoreGraphics/CoreGraphics.h>

int main() {
    CGDirectDisplayID displays[2];
    CGGetOnlineDisplayList(2, displays, NULL);

    CGDirectDisplayID st;
    CGDirectDisplayID nd;
    if (CGDisplayIsMain(displays[0])) {
        st = displays[0];
        nd = displays[1];
    } else {
        st = displays[1];
        nd = displays[0];
    }

    CGDisplayConfigRef configRef;
    CGBeginDisplayConfiguration(&configRef);

    CGConfigureDisplayOrigin(configRef, st, 0, 0);
    CGConfigureDisplayOrigin(configRef, nd, (CGDisplayBounds(st).size.width - CGDisplayBounds(nd).size.width) / 2, CGDisplayBounds(st).size.height);

    CGCompleteDisplayConfiguration(configRef, kCGConfigurePermanently);

    return 0;
}