Unfortunately this is not possible at the moment, even though we are planning on adding a playlist feature that way you will be able to assign more than one video for assignment, but right now it is not available.
Therefore, we recommend to change the title and add a (Part 1 of 3, Part 2...). The students will know how many videos they have to watch "per assignment".
Here is a video that explains how to change the title and another to organize the assignments:
Organize. Click here to watch it on Youtube