一山不容二虎 "This town isn't big enough for the two of us." (Eliot's suggestion is a pretty good equivalent.) "There can be only one." (Highlander movie quote) There can only be one top dog. ‘When two ride on a horse, one must ride behind.’

**This town isn't big enough for the two of us.** (A more modern one: There is only one alpha male... But it sounds kind of lame.)