What are the old man's two pieces of luck?

At the end of the story, the narrator explains that "There was nothing to do about [the old man]" on the bridge. He has tried to encourage the old man to get off the bridge and walk toward where he might find some assistance, but the old man has run out of energy and cannot bring himself to walk farther on. The old man only seems to worry about his animals, the ones he had to leave behind when he was evacuated, and not at all about himself. He says that, because he knows no one in Barcelona, it will not help him to walk in the direction of the trucks that way, and he doesn't seem to realize how much his own life is in danger. Any place would be better than the one he's in.
The narrator says that it was a "gray, overcast day with a low ceiling," and so the Fascists' planes were not up in the sky. "That and the fact that cats know how to look after themselves was all the good luck that old man would ever have." The two pieces of luck, then, are that the enemy planes aren't flying yet—and so the old man is still alive for the time being—and that his cat will probably survive the coming destruction. The implication, of course, is that the old man certainly will not, and his death will likely be a brutal and bloody one.

Although the soldier at the bridge tries to reassure the old man that his animals will be safe—for instance, that his pigeons will fly away since he left their cages open—it becomes clear by the end of the story that the soldier believes the old man's case is hopeless.
He thinks cynically to himself that the old man's two pieces of luck are that his cats will be able to fend for themselves and that it is too overcast for the German soldiers to bomb the bridge and surrounding area this day.
That's a very slender reed of hope. The old man doesn't have much future ahead of him: he could be bombed tomorrow. The story shows the ways war can devastate the lives of simple, innocent beings like the old man, and his animals, who are the collateral damage we might not even consider.

In Hemingway's short story, the old man has evacuated from his town of San Carlos and walked twelve kilometers to avoid artillery fire. Unfortunately, the old man is exhausted and cannot continue to walk away from the bridge where the advancing enemy forces will cross at any moment. The old man laments to the soldier about the fact that he had to leave his cat, two goats, and eight pigeons behind in San Carlos and worries about their well-being. The soldier attempts to comfort the old man by offering him words of encouragement, and the old man feels confident that his cat will survive without his protection. However, the soldier knows that there is not much hope for the old man and the rest of his animals. At the end of the short story, the soldier says,

It was a gray overcast day with a low ceiling so their planes were not up. That and the fact that cats know how to look after themselves was all the good luck that old man would ever have.

Essentially, the only two pieces of luck the old man possesses concern the fact that the Fascists' planes are not in the sky because of the overcast weather and the fact that cats can take care of themselves. The soldier realizes that the enemy forces will eventually cross the bridge and murder the innocent old man as they continue to advance through the country.