Perhaps this only works in my brain, but I'd like to think of having 4 classes of windows per monitor. These window states are: floating, full screen, "stuck" to left side, and "stuck" to right side.

The size of a floating and full screen window works pretty much as I'd expect it to work, but I have problems with the other two. In my mind, width(stuck to left) + width(stuck to right) should = width(monitor). This currently passes, but unfortunately... width(stuck to left) = width(stuck to right) = width(monitor)/2. I'd like to be able to readjust these so that they didn't have to be equal. 

Now, I'll explain the way I think this could work in a way that is easy to understand at least after you see it work.

I think you could just resize the width of a window that is "stuck" to one of the sides and it should resize all windows that are "stuck" on this monitor. In other words, if you have windows A and B "stuck" to the left, windows C and D are "stuck" to the right, window E is full screen, and window F is floating. Making window A 30 pixels wider makes windows A and B 30 pixels wider, windows C and D 30 pixels skinnier, and no effect to windows E and F.

Perhaps a UI hint could be something like a tab stop that is always visible at the top or bottom of the screen that indicates where the split currently is. Another option might just be to have some sort of vertical line that shows up when resizing a "stuck" window that may indicate this affects more than just the one window. Another potential option is something like a vertical separator that is visible on the edges of the "stuck" windows.

Obviously, adjusting this for one monitor should not affect any other monitors.