Two runners at a base

[BretMan]

FWIW...here is the MLB case play that covers this.

PLAY—Runners on second base and third base with one out. The runner from third base (i.e., the lead runner) makes an advance toward home and is caught in a rundown between third base and home plate. Believing the lead runner will be tagged out, the runner at second base (i.e., the trailing runner) advances to third base. Before being tagged, the lead runner runs back to and beyond third base toward left field. At this time, the trailing runner has passed the lead runner as a result of the lead runner’s actions. As a result, the trailing runner is out and third base is unoccupied. The lead runner is entitled to third base if he returns to touch it before he is out, see Rule 5.06(a)(1), unless he is declared out for abandoning the bases.

This interpretation was just added a couple of years ago, in response to a similar play and debate about whether or not running past third base like that constitutes "passing a runner". The case play clarifies that it does.