I just ran into the same issue. It appears that the 24c, with the Mixer knob set to Input, sums the two inputs in mono and then outputs the same mono signal to both left and right outputs. This was a big disappointment to me. The Scarlett 2i2 has a mono/stereo button that I believe makes it behave as you and I expected the Presonus 24c to behave. As a Scarlett 18i20 owner, I now wish I'd stuck with Focusrite when adding a low end box for my other machine.
I had to use Loopback on the Mac to send input 1 back out to left output and input 2 back out to the right output, then turn the Mixer knob hard right. Of course that totally gives up the low-latency purpose of having input monitoring on the box.
Very disappointed with this thing.