As a player who has played both sides: The first time you will see an opposing player is on Nar Shaddaa on the promenade. You will not see them in questing areas as you quest in DIFFERENT areas. The first time you will meet an opposing faction player in regions where world PvP can occur will be on Tatooine. NB: If on a PvP server you will be automatically flagged on entering contested zones so watch your back on a PvP server. (Even tho entering a questing area on Nar Shaddaa will flag you for PvP you won't be attacked). As was stated above, Taris is visted at two different level brackets for each faction. It is the same for Balmorra. When the Empire is on Balmorra, tPhe Republic is on Taris. When the Empire is on Taris, the Republic is on Balmorra.