cannot apply track matte to a second layer

hi

i have a footage showing office's screen which displays some statistics, etc.

I masked out that screen and added a different footage within mask's boundaries and now the screen displays some football match instead.

Now i want to add another footage right after football footage - to imitate switching between tv channels (i guess it's also called channel surfing?). In other words, i want that screen to display a football match, then some news report, then some documentary, etc.

So i added 2nd footage to may composition and placed it under football layer (refer to printscreen)

I selected football and 2nd footage  and made a precomposition. then applied Alpha matte and set track matte as parent. Did not work - only a white rectangle was displayed instead of footage.

Then i tried to duplicate track matte and placed it right above 2nd footage and set it (track matte duplicate) as parent - was thinking that each new footage may require separate track matte. did not work either...

if somebody could take a look at the printscreen and gie advise it would be awesome.

By the way, i managed to display 2nd footage by applying Alpha inverted matte and setting it as a child to track matte, but its not quite right because i have to meticulously adjust 2nd footage's size manually to fit into office screen's frame and its really hard to do