no sound from the first track while recording the second

see i have been recording one track then when i try to record the second the first one does not play how can i get it to play while recording the second? need help please

Audacity > Preferences > Recording > [X] Overdub (select).

we wrote a whole class on this.

http://manual.audacityteam.org/o/man/tutorial_recording_multi_track_overdubs.html

Koz