1)    One who finishes his prayer and sees that there is someone else praying behind him, should not take the three steps backwards if he is within four cubits of the other person or if the steps backwards will take him to within those four cubits. (Shulchan Aruch and Mishne Berurah 102)

2)    This law applies even if the person behind him started to pray after him and prays for an extended time. Nevertheless, this is not considered as if the one behind is “trespassing” on the one in front, as actually when the man in front takes his three backwards steps it is he who is moving into his friend’s area. (ibid)