Python plot two heat maps at once using squares divided in 2 halves

I am trying to generate a heatmap where each rectangle is split into two pieces and can be colored differently. Essentially, I have two 20x20 matrices that I want to be able to plot on one Heatmap, similar to what is done in this here except I want to divide into 2 rectangles rather than triangles.

Any suggestions on how to do this? It's not obvious there's an easy way to do this but I was thinking of using a hacky solution where I use pcolormesh to draw a 40x40 grid and then somehow plot my points onto that. I'd like to be able to color the outside of single rectangle in a dark line and then use a thinner line to divide the rectangle in 2.

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