There is no reason for the Ninja's 'Dream within a Dream', and Assasinate to use two seperate buttons.

Assassinate can only be used within 15 seconds of using 'Dream within a Dream'. 'Dream within a Dream' can only be used once every 60 seconds.

So there is no point at which we need both these buttons present on the bar. The 'Dream within a dream' button should become Assassinate for 15 seconds after it is used.