It says "to maneuver THEM", which is plural. Seems to me the intent is that you take a -2 when using any number of them. I mean, the benefit doesn't go up for using more, so why would the penalty? Don't be silly guys.
"Are you using your Skimmers?"
"Yes, I'm using them."
"The fragile balance required to maneuver them gives you a -2 to your physical limit."
"Ok, cool."