I created a layer group for the purpose of using single strokes of the eraser tool to simultaneously apply the same fractional erase percentage to the same areas of two layers, and was told that "painting on layer groups" wasn't possible.

Although it makes sense that with various image editing capabilities, there could be too many possible operation combinations to guarantee deterministic results when applied to a layer group, an action performed with a simple tool like the eraser should be able to be iteratively applied to each layer in the group as long as certain constraints are met.

Applying such limitations in more situations than necessary seems overkill if it is merely an artifact of current implementation details.