The way of the Warlock Part 3 - no joint attack prompt, cant get any hits (video)

[SnowWhirl]

 

im not sure what i am doing wrong or if i just can't complete the quest. i watched a video of somebody else completing this ( https://youtu.be/T-r4ltvZa7Y?t=2h35m10s ) all they did was press spacebar and it completed instantly, i tried to copy the timing manually and it still didnt work. i cant find any other video on this quest and there is no information on what i am supposed to do. i've used tether blade at least 100 times with varied timing and have not once gotten it to count. i made a video above and would appreciate any advice. I have tried restarting the game and leaving and returning as well.