I'm having issues trying to create a full round fillet on a rib feature mounting boss that will be used in a plastic enclosure.

I was able to full round a face shown below on separate body that is nearly identical in size. However it fails when the adjacent face is also selected. These are the two faces that we would like to create a full round fillet on the ribs.

I've attached the model for reference. Thanks in advance.

Note that I initially used a rib feature and ran into this, then tried using extruding boxes and separate bodies without any luck.