The two-beep occurs 2 seconds or 48 frames before the picture start.  It is a 1KHz sine wave, one frame or .042 seconds in duration. 
When using a standard SMPTE film leader the two-beep occurs when the countdown reaches 2.  In preparation for a film or video mix, all audio tracks should have a two-beep.  This enables the mixer to hear if all the tracks are in sync with each other and the picture at the start.  If all is well one will hear one LOUD short beep.  Any synchronization problems will cause the beep to be lengthened or even multiple beeps to heard.