In traditional RTS, the Shift key is used to add or remove units on the current units selection (Not group, selection).

If you click Shift on a unit that is not selected, then it adds it to the current selection.
If you click Shift on an already selected unit then it removes it from the current selection.
Same on the UI.

In AoD, you have to use Shift to add and Ctrl to remove from the current selection, which makes it a bit akward and clunky when you have to manage your units in a fast pace gameplay.

Is it possible to have the same button to do both functions, or at least an option, even something to change in a config file manually or is there an underlining technical limitation preventing from having the same key do both functions?