IMO, using different key binds for Tab Target(TT) mode and Action Combat(AC) should not be required if possible. Currently, you cannot bind the same key to Guard for TT and AC. I would like the guard key bind for both modes to be on the right mouse button. I think that many other people would as well.
In TT, the right mouse button is used to have your character move in the direction that you are pressing (WASD). This function could be assigned to the left mouse button, but that button is used to free look around without moving your character. Neither of the mouse button functions in TT can be assigned to other keys.
If players could choose to rebind all of the above options freely, then block could be allowed on the same key in both TT and AC. If I had this option, I would set the left mouse button to allow me to guide my character in TT (the function that is currently permanently on the right mouse button in TT); and, set holding the middle mouse button down as the way to freelook (the same function that is currently on the left mouse button in TT).
A possibly slightly easier way to program the above (I am not a developer and I'm guessing at this) could be to set the freelook option as a key bind, provide a checkbox to move the function for the right mouse button to the left button in TT, and allow both guard modes to use the same key.