In Wallace Stegner's Crossing to Safety, Sally was in Boston and Larry was in NM before they were summoned to the Lang's house. Stegner never said why they were apart. It's driving me loco.

I believe what you are referring to is the fact that Larry spends his mornings writing about a novel set in New Mexico and his afternoons writing about a setting in Italy. Thus.... he isn't really in New Mexico, he is merely imagining himself there.