rafiano said:solution.....use the IR trigger..
A suggestion, if you own a good flash like 580ex or 550ex on canon system. Or SB800 on Nikon.
Set your flash (e.g. 550ex )to manual mode, and set it to the lowest brightness (this is to avoid the flash (e.g. 550ex) affecting the overall lighting). Tilt the flash direction away from the subject (again to avoid 550ex afftecting the lighting). The 2 slave flash will be triggered by your master flash without connecting the cords.
*built in camera flash or 420ex on Canon system will not work because it fires a pre-flash that will trigger the slave flash prematurely