Not knowing the specific brand/model etc.. I cannot give specific recommendations, However most earbuds I have seen have a 3.5mm stereo (3 circuit) phone plug. Paralleling here will half the impedance seen by the earbuds amp so that may be unworkable for some systems, however if it is still within the operating range of the amp it will give what you desire.
If paralleling the earbuds won't do then a patch cord or adapter should be able to be connected at your mixer so the dead channel (earbud) gets the signal sent from the other (working) earbud.
Last edited by bassmeknik : 01-31-2014 at 10:57 AM.