Consider having the upped one come into play with its shadow. Being able to create 2 shadows isn't really that much better than being able to create 1 shadow, since you can only make one per turn.
Think about it this way: For both assassins, if I don't have CC on the turn it comes out, I need 2 CC the next turn. I probably will not draw into 2 CC. IF I don't have enough CC for the first 2 turns, then Agile Assassin's ability MIGHT matter, but probably by this point, I have already given up on killing the assassin.
Also, in some ways it's actually better to have 1 shadow out than 2. With 1, I can immolate/oty/mutate the shadow, buff the assassin, and then put out another shadow in my turn.