I believe there should be a third mod.
Reasoning: You could act as supervisor over the two other graphics mods (due to your seniority and experience with the position already). Also, there is really no reason to not have another moderator. The more help the merrier!
The absolute worst thing that could happen is one of them have to step down and you come back in to help the remaining mod. (Its not like they can ban)
Gl with you decision.