Understanding the Force of Two Cubes

[Woopa]

Homework Statement

Two hollow cubes of side 25 cm with one face missing are placed
together at the missing face. The air inside the solid
formed is pumped out. Determine the force that is necessary to
separate the cubes.

Relevant Equations

P=F/A

The solution is :

The pressure inside the solid is zero and outside it equals atmospheric pressure, 1.01 × 10^5 Pa.

Thus, the force is given by: F =pA = 1.01 × 10^5 × (0.25)^2 = 6.3 × 10^3 N

I do not understand why only the area of one cube is used in the solution, should it not be the area of both cubes e.g. Area of 2 cubes = 2 x (0.25)^2?

 

[haruspex]

This is analogous to a rope being pulled with force F at each end. Is the tension F or 2F?
Just consider one cube. What forces act on it in the direction normal to the missing face?