The idea is pretty simple: to be able to select either padding/margin in the UI (as you can currently do) but then to be able to add a value which effects all parameters selected.
So a minimal change but I feel it would make the process of adding funky values to different areas a bit more intuitive and streamlined. You would just have one value box in the centre which would show the value of the highlighted areas of margin/padding. To adjust some areas but not others you would need to select or deselect as appropriate.
Hope this makes sense...please don't make me do a diagram.