I think canceling the F-136 would be penny wise and pound foolish. Anyone who remembers the introduction of the F-100 engine remembers it was beset with problems, many never solved until GE started competing for engine business. Also, the F-135 has it's problems and it is over cost as well. The competition will keep them both honest, and IIRC, Britain prefers the GE engine with it's greater thrust capability for their STOVL variants for obvious reasons.
I've also read reports/presentations where GE talked about growth versions of the F-136 with a thrust rating of 60,000lbs. I'm assuming that's with a new/uprated hot section and a new fan. Still, that engine has a lot of potential designed into it and it would be a shame to shut it down now. If they need to save that much money, just trim it from the entire program. Because I haven't any doubt, that in the long run, two engines competing against each other will save more money than a monopoly supplier would save with the entire run.
Also, having the two engines would keep some airframes in the air in the event there is a major problem with either of the engines.