I don’t think you can repeat multiple background images and keep them sized to 50% of the viewport (you can’t have the background-color in the shorthand anyway).
I would just use 2 pseudo elements and have complete control.
No you can’t have the background color in that multiple shorthand rule.
You can have multiple background images and place them where you want but you can’t restrict their size. If the images are repeated they will overlap if anything else is on the way.
You could repeat something down the left and a different image down the right bit you could not make them meet in the middle:)