Daisy chain the output from one laptop to the line-in or mic on the second laptop. This is safer than other methods.
You'll need a 3.5mm male-to-male stereo audio cable to chain them.
See some more details here: http://www.reddit.com/r/techsupport/comments/189rfa/how_do_i_put_the_audio_output_of_two_different/