You can now select more than one vessels, and playback their animated track for a specific timeframe!
Open a Vessel's InfoWindow and select the "Show vessel's track" option.
A "Track Options" toolbar appears at the bottom of the map. Use the "Add Vessel" button in order to search for the second vessel of your interest. Proceed this way for the rest.
Click "Play Track" to see the animated tracks simultaneously.
Select the "time period" of the track (from 1 to 60 days depending on your subscription plan) and change the playback speed using the respective controls.
Note that changing the length of your default track can also be set at your Preferences page.