Will not happen - PA has a "sore loser" law that prohibits the loser of a primary from running in the general with the other party or as an independent.
That is not all of Arlen's problems. The PA primary is closed - no 'Rats or independents allowed. A recent poll of GOP voters here showed a 66-26 preference for bouncing Arlen. He is trying to get the state GOP to open up the primary.
Actually, the smart thing for him would be to turn 'Rat in name as well as deed. If he doesn't, he is toast next spring.