BTT 3.182 (1361) on macOS 10.14.6 on MacBook Pro 13-inch, 2017:
When the macOS dock is anchored on the left side of the screen, the actions to size a window to 2/3 of the left and a second window to 1/3 of the right result in an overlap of the two windows. The same is true for 2/3 on the right and 1/2 on the left. I suspect the screen positioning calculation is not factoring in the space used by the vertically oriented dock.