Shenzhen is a better case example. SHA has too little flights to be of competition to PVG for connections.

Shenzhen runs connections to Hong Kong airport. People fly into SZX then connect to an international flight from Hong Kong. Or vice-versa.

People still keep on doing it because they want to avoid the flying into Hong Kong tax.

The business decisions of the airlines to maximize profit could cause a similar thing in Beijing. Don't want to spend 2 hours transiting through Beijing? Fine pay us an extra 300 rmb and we will let you connect in the same airport. Airline bosses are probably already planning their pricing structure to advantage of two MAJOR airports in Beijing.

Maybe I am pessimistic because I see the numbers who fly to SZX just to get to HKG. But I don't think I can trust the airlines not to take advantage of this.