Absolutely "Yes," you may have more than one Influence Character as your story progresses along. There are two ways to do thishe first is to hand-off the duties between players. This usually happens at act changes but need not. An example of this type of multiple ICs can be found in A Christmas Carol by Charles Dickens. The "Ghosts" collectively are Scrooge's Influence Character. You'll notice that the different ghosts never appear together. This is important to keep from confusing the audience about their function in the story.

The other way to have more than one Influence Character is to have the Influence Character represented by a group. The tricky thing about doing this is that each of the players representing the Influence Character MUST share the same world view or condition and thus have the same impact on the Main Character. For example, in The Incredibles, Helen Parr (Elastigirl) is Influence Character to her husband's Main Character. Their relationship is mirrored by their children, Violet and Dash. In essence, both the MC and Influence Character throughlines are explored through two players each. Though this example show two MCs and two ICs, you can easily have one Main Character with multiple Influence Characters.