Yes, this is mentioned in the Introduction to the Third Edition, in which Le Faye states:

When the sisters were apart, they wrote to each other about every three or four days--another letter begun as soon as the previous one had been posted. There is always a first letter from Jane telling Cassandra of the journey from home to the destination; then a series of letters talking obout daily events at the other place; and one or more letters planning the journey home (p. XV).