They're playing crazy eights and the best part is her ass (which is just as smart as her) is going to win no matter what!
The second possiblity is Unlike the first one that is relies on magic, bacsically like the lady sawn in half. Blackberry and Razberry still get along and cooperate to look like a normal girl when stacked. They are still completely independent from one another for life functions, but can connect like a magnet. At the stumps, it's smooth skin with no jagged edges, if you were to take an xray of her guts you's see that the only spot that was supposed to connect are her spine, and the first part of her small instestine, no guts are divided. It's not really like a worm hole but rather an asymptote.