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I read and interpret the condition by different way: two spherical solids are taken out by cutting from a solid cube
in a way that their centers are located on the 3D (=longest) diagonal of the cube.
(This condition provides the maximum radius and maximum volume to each of the two spheres).
Then it is clear that these spheres touch each other at the middle of the 3D diagonal of the cube.
The length of the longest 3D diagonal of this cube is cm.
If "r" is the radius of the sphere, then = cm.
Hence, r = = 4.12 cm.
Then the volume of each sphere V = = = 292.8 cm^3.
Solved.