Why is there only one の in 牧師から信徒への手紙? > **** **** "a letter from a pastor to the congregation" \- weblio example shouldn't it be > **** **** ? It seems to me that and are modifying independently. Is this some kind of rule where shouldn't be repeated in such cases?

AB and AB are treated as one set, and only one is necessary. I think this is an exception you need to remember. (I have mentioned this in this answer.)