It works just like it says. 2 of your units may move through one void space (with out a ship). You can go from planet a - void - void w/ship planet b, only 2 units, only 1 void space. Yes your example is legal for 2 ground units
Just remember you move ships first, then troops along a legal path. Only one adjacent area may activate and move into the activated area.
Thank you so much =)
Forbidden Stars is such an awesome game