Here is the other unfortunate reality: Let's say that Pennsylvania and Maryland decide to make the very agreement that you first mentioned. (or you could say Virginia and Maryland)
PA goes from 'free' to 15, and Maryland goes up from 10 to 15, with Shall-issue CCW. PA gets shafted, and MD gets a good much better deal than they had before. PA and MD gain CCW reciprocity.
Along comes the first mass shooting in MD with a 'high capacity' magazine... Governor O'Malley Hogan of Maryland and the legislature decide that they erred, and adjust Maryland's laws back to 10 rounds. Maybe they keep shall-issue, maybe not. Now they know what tactic works, they tell PA to go down to 10 rounds along with them or else. The PA public and legislature say F*** You, and MD revokes reciprocity. In response, the PA lawmakers repeal the state magazine limit... only to have Governor Wolf (D) veto the bill in the interest of 'public safety.'
Now both states are screwed.
Unfortunately, this would happen almost certainly, exactly as I described.
Kaso