I love the half/half screen mode when you snap a window to each side of the shell :)

It's so useful for coding help and researching while writing.

However, after I've looked up the thing I need right now, I often wish I could make it 30/70%, or even less.

So how about it? The windows are already snapped to eachother and you remove the individual resize handle now (->| and |<-). How about you add in a |<->| (or similar) cursor then to simultaneously resize both of them vertically?

Another option might be to do a global modified mouse click on the window to toggle 70%/30% - 50%/50%, so the clicked window would get the majority on first click, and it would return to 50/50 on second click.