If two psions fuse and one is a shaper and one is an egotist. Is the fusion both? So I could use a power stone of either?
The fused being knows all the powers you and the other creature know, [...] Likewise, all feats, racial abilities, and class features are pooled
So yes, the fused being has the class features associated with being an egotist as well as the class features associated with being a shaper. That includes adding the egotist-only powers and shaper-only powers to its power list, which is the requirement for using a power stone.