You can't do this directly with the inset tool. Instead I would do this using loop cuts.
Hover near one edge of plane one and type R to start the cut. Type 2 or scroll the mouse wheel to get two cuts. Left click twice to finish the cuts:
Immediately scale the loop cuts along the axis they're perpendicular too, in this case, the x axis, by typing SX to scale. Use the mouse to scale the loop cuts into position:
Now do the same thing, but hover along one of the edges perpendicular to your first choice. After adding the cuts and scaling on the other axis (In this case Y) you get this: