I booked an American Airlines ticket Newark --> Philadelphia --> Pittsburgh since it was cheaper than direct Philadephia --> Pittsburgh. I would like to simply get on at Philadelphia (I am traveling from central Jersey), but I assume my ticket will be voided if I don't show up in Newark. Is it possible to get the airline to divide my ticket onto two bookings so I can cancel one?
What you are trying to do is "hidden city ticketing" and it's forbidden by AA's contract of carriage, so I would not expect them to make it easy for you to accomplish this.
I would think that if you try to split the itinerary, they will compute the total fare for one-way trips EWR-PHL and PHL-PIT and then charge you for the difference above what you have already paid, perhaps in addition to a change fee of $100-$200 if your original ticket was non-refundable. So this is even worse than just changing your itinerary to PHL-PIT.