During a long rest if someone is fully rested, can they keep watch longer than 2 hours?


So I am currently running a game with 3 PCs, 2 of which are Elves and 1 a Human. I am just trying to calculate the most efficient way to run watch shifts during long rests since both of the elves only need 4 hour meditation to be considered fully rested.

In the PHB pg 168 in the section about long rests it states that (emphasis mine):

A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than 2 hours.

Now my question is, once a character is considered fully rested and no longer needs the “long rest” are they able to keep effective watch for longer than a period of 2 hours? So say that the elves both finish their 4 hours, can they now keep a vigilant watch for the other 4 hours the human PC needs to rest?