Where do you see the worm drive ?
I see a motor in the top left corner which seems to have a planetary gear setup in integrated. The metal gear at the motor’s output transfers motion to the small black gear located in the upper central position of the carriage.
I think what can be interpreted as a worm drive is actually a spring.
Because with a worm gear, movement could only be transferred in one direction, from right to left.
A cam at the top could, depending on its position and shape, allow both extruders to engage simultaneously, as I described earlier, or engage only one of the two extruders at a time.