The same way when automakers tried a similar game and then.. the government stepped in, slapped the tar out of them with regulation, and told them to play nicely with third parties or lose the ability to do business.
Or when utilities had natural monopolies were preventing competition and.. then the government put strict regulations on them, set pricing rules and allowed significant control of them.
The answer is that if the two dominant parties have managed to keep out competitors and basically engage in collusion, the government should correct the situation.