Thank you for reaching out.

This is brown spot, usually caused by too much water.

Remove plant from current soil and replace with cactus/ succulent soil.  If roots are mushy, trim away mushy parts.  Wait 5-7 days to water.  Water only when dry.  Be sure pot has drainage holes.  The plant needs sunlight- east or south facing windows.

If plant is too far gone, you may lose it no matter what you do.  I’m sorry.  Succulents do better dry.  Good luck!

Thanks for the reply! I should provide back story, I’ve had this succulent for about 4 years, it has been healthy until about 5-6 months ago. I also have a jade in a similar boat, it was healthy and thriving and around the same time it started showing signs of stress. I googled a bunch of stuff, and ultimately I *think* they both got sunburned, I had reshuffled some furniture and the surface these sit on went from indirect south sun to fairly direct south sun in the summer/fall, that's when I realized it might be too much direct sun. Now that it’s winter I’ve left them in that window (I live in Mpls so the houses are close together, any direct sun is only late in the afternoon and only if it’s out). 

There are a couple of things you can try. Understand brown spot is a fungus and it does spread.  There are fungicides you could try.  You are looking for something that treats Septoria leaf spot.  If you choose this route,nfollow instructions on the package.  I Am not allowed to suggest specific chemicals, but you can google fungicides for brown spot.

You also could try removing all of the affected leaves and trim down the stalk to a definite node.  Let it dry out really well.  If you do this do not apply fungicide.  Put plants in bright area.  I agree the south sun in the summer may be too much, but where you have them sounds good.  Just be sure the area isn’t to humid.  I still would check the wetness if your soil and consider new soil.  Sometimes these fungus gets into the soil also.

I would isolate these two plants from others for the time being.

Be aware that this might not work.  I would try cutting back and removing leaves first.  If they come back and still show signs of leaf spot then try the fungicide.
all you can do is try.  Sorry I don’t have a total solution for you.