One of the rules is, that he has to make direct eye contact. Yet hes able to geass about twenty people at the same time in Episode 1 from Season 2. He even is able to do it later, too.
Can Lelouch use his geass more than once?
The ability cannot influence the same individual twice. Geass does not have a time limit as shown when Suzaku Kururugi was commanded to live, which went on until the end of the series. Lelouch could initially toggle Geass activation at will, but subsequently loses this control and suffers from a runaway Geass.
Does Lelouchs geass get stronger?
The Geass-in-both-eyes thing is just how the ever-increasing strength of Lelouchs Geass manifests. C.C. mentions on multiple occasions that his Geass will grow stronger and stronger, to the point that it cannot be controlled (even by the special contact lenses).