You are right in identifying the subtlety of the title. The first jilting is of course easy to identify, as it refers to the jilting that Granny Weatherall suffered so long ago and which seems to dominate and impact her still, even though it is years later and she is on the verge of death. Although she appears to be defiant and wants to "get back" at George, the man who jilted her, we can detect that really there is an immense sadness over her loss that her subsequent life and marriage did not entirely erase:
Yes, she had changed her mind after sixty years and she would like to see George. I want you to find George. Find him and be sure to tell him I forgot him. I want him to know I had my husband just the same and my children and my house like any other woman. A good house too and a good husband that I loved and fine children out of him. Better than I hoped for even. Tell him I was given back everything he took away and more. Oh, no, oh, god, no, there was something else besides the house and the man and the children. Oh, surely they were not all? What was it? Something not given back...
She seems to want to strike back at him and to insist that she had a good life in spite of his jilting her. Yet she clearly senses that something was curiously absent from this "good" life.
Lastly, however, as Granny waits for death and asks God for a sign, some argue that Granny receives her second, and symbolically far more important, jilting:
For the second time there was no sign. Again no bridegroom and the priest in the house. She could not remember any other sorrow because this grief wiped them all away. Oh, no, there's nothing more cruel than this--I'll never forgive it. She stretched herself with a deep breath and blew out the light.
Porter seems to be suggesting by this interesting ending to her story that we ultimately are all jilted in death. We die alone and this solitude is greater than any loss we may have known in life. However, in spite of this second jilting by God, note how she suggests Granny Weatherall's immense strength of character in accepting this unjust fate in the face of this ultimate jilting.