The Shulchan Aruch (102:1) rules that it is forbidden to sit within a radius of four Amot approximately 6-8 feet of somebody praying the Amidah. This applies to all directions: one may not sit in front, in back, or to the sides of a person reciting the Amidah. Since the Shechina is present when a person recites the Amidah, it would be disrespectful to sit within four Amot of that person.
However, Chacham Ovadia Yosef, in his work Halichot Olam (vol. 1, p. 146), rules that a person who is praying or studying Torahe may sit within four Amot of somebody reciting the Amidah if he had been sitting in that seat before the person began the Amidah. For example, if a person recites Korbanot, Pesukei De'zimra or other sections of the prayer service, and somebody begins reciting the Amidah near him, the first person may remain seated. In fact, Chacham Ovadia writes that one should not disrupt his study or prayer to stand when somebody begins reciting the Amidah near him. One may not, however, sit down to learn or pray near somebody who had already begun the Amidah.
Summary: One may not sit within four Amot (6-8 feet) in any direction of somebody reciting the Amidah, unless he studies Torah or prays and was sitting in that seat before the other individual began reciting the Amidah.
