Bevel 2 edges

Hi all, I was wondering how I can bevel 2 surfaces to create an arc between them on the horizontal faces of my object. I tried the bevelsrf command, however, the problem lies in already having a bevel in the vertical dimension of the object. The layer in red is what I want to fix and the object is a Rubik’s cube edge piece. Any help is appreciated.

Cubes.3dm (2.1 MB)

Cubes.3dm (2.4 MB)
Fillet surface can help you out

Thank you for the file, Matthew! I was wondering what I would need to execute the command successfully, as every time I select two surfaces on my object, the command fails.