As you said, we only do 2 EVs currently. However, we do have more than two entities working in parallel. For a hypothetical example, during a joint Space Shuttle and ISS EVA, there would be EV1 and EV2, but there might also be the Space Shuttle robotic arm and Space Station robotic arm (each run by at least one crewmember). Also, there may be a crewmember inside coordinating the EVA (reading procedures) who may have some tasks that need to be done to support the EVA, and there is commanding done from mission control. So that's 6 entities.

From an EVA planning perspective, we generalize that to three entities: EV1, EV2 and everyone else. That is, we have three columns in our Word document, and steps in the "everyone else" column will specify who is performing them, i.e. "Station Arm: translate to hardware pickup location"