Some pairings feel inevitable. "Friends" had Ross and Rachel. "The Office" had Jim and Pam. Elon Musk has SpaceX and Tesla.But a deal between Musk's two giant companies could end up looking more like Frankenstein's monster than a Hollywood love story. And investors might struggle to understand and value it.The SpaceX-Tesla tie-up isn't a shoo-in, but both sides are officially considering it. That raises questions about what's gained from a merger. Combining the companies turns Musk's two jobs into one. Thei ...