Why Lelouch needs geass in his two eyes? In other question, it was clarified that Lelouch's two eyes Geass is to show the growing of Lelouch's Geass power and to stop his father's plans. what left of it to understand is, besides the "growing power" is there any difference in Lelouch's Geass demand power from before? I mean, in Mao case for example once his Geassh reached the limit and turned his two eyes he couldn't control it anymore, but in Lelouch case, we don't see any difference in the using of his Geass, so except the inevitable Geass growing power was there a difference in Lelouch's Geass using when he has 2 Geass eyes?
I have a theory about this.

The Geass consists in three stages: 
Normal stage -> Lost control stage -> Both eyes stage.

The point with third stage is that, only in this stage, the user is able to complete his/her contract and become a Code. Lelouch and any other Geass' users will not show any difference in his/her power level or new abilities, since this stage only remarks the new capacity of the user to become a Code.

After watch the anime twice (and Akito once) I think this makes sense and there is not (as far as I remember) anything that contradicts it.