Technically they could co-exist. But internally (and externally, as it is seen by the user) they are both toggling modes.
So what happens when someone goes from Normal, Alt+F10, Ctrl+Up, Alt+F10, Ctrl+Up? Should the second Alt+F10 goes back to the normal mode, or simply get rid of the UI?
In other words, everything is possible, but I would rather this is tackled first as a design decision agreed on. Then later we/I can worry about the implementation.
Personally I stumbled on this issue by chance. When you think about it maximize window is a step between normal window and full screen , so maybe they should be switchable but different shortcuts make this suggest different modes.It would be nice if they used the same shortcut to switch from maximized to fullscreen (Ctrl + Up x 1 = maximize, Ctrl + Up x 2 fullscreen). That's my opinion on the subject.