I enjoyed the cell shaded look. Mechanically speaking, I enjoyed the core premise of "only being allowed to be so far apart for X seconds". I think there's a lot of promise there. I'm not sure an arcade platformer is necessarily the best choice to showcase it, though it makes sense given the time constraints.
I think if you were to pursue this concept further, you might want to explore a puzzle/adventure platformer, a la Portal or Q.U.B.E.
For example, one puzzle might be need to run into an area to stand on switch, but in order to get there, one character must walk outside of tether range. This in turn might activate a conveyor belt or a fan, which would enable the second character to get back to them in time, while normal walking speed would not suffice.
Hope to see you keep iterating on this idea!