I'd like to assign two shortcuts to the undo action in blender 2.8 (ctr+z and middle mouse click) but I failed as it seems only one at time is possible. Is there any way to do so?
There is a way, but it's a very bad idea
You should not do it. Middle click is used too much to practically change it and retain important functionality in Blender's keymaps.
You would have to add a new entry here in the Preferences(F4 -> P), Keymap tab, under Screen, Screen(Global):
But you would also have to search for and remove all the conflicts:
That would break stuff. This would be a very bad idea.