[paranoidrobot]

There can be multiple aircraft on the runway at multiple times, so long as there's no conflicting operations.

It's used quite regularly at any moderately busy airport to reduce delays.

Examples where I've seen/heard it used in ATC comms:

Aircraft departing in sequence. eg A, then B. So: "Flight A (already lined up on runway 35L), cleared for takeoff runway 35L." (some short time later while Flight A is still in the process of taking off) "Flight B, runway 35L line up and wait"

Aircraft departing after an arrival. eg "Flight A cleared to land, 35L", and right as flight A has crossed the threshold: "Flight B, runway 35L line up and wait"

Also, as mentioned by others, when there are other movements to cross a runway between departures. eg "Flight A, at Foxtrot 5, cross Runway 35L. Flight B, runway 35L line up and wait"