Unfortunately not, it is only possible to have one video box on any one sign. However, if you have multiple videos that you would like to display, you can add these to the same playlist. This means that when you assign the playlist to a sign with a video box, the videos will play one after the other within the sign.Alternatively, you can create multiple signs, each containing a video box with a different video playlist, and link these in a sequence.
More information on creating sequences is given here: