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Thats the problem with backlighting. No way to see the units directly without destroying the effect. I like the idea of the tethered unit but requires me to buy another 580.
If you're using one 580 for the back light and one 580 for the main (front) light, you could raise the front light, set to Master and on an ETTL cord, high enough that some of its light spills over the subject and onto the slave 580.
If you're using both 580s as back lights for a sillouette effect, and controlling them with the pop-up flash, then you may be able to move them outward until the sensors can see the camera, and use the swiveling heads to point the beams at your subject's back. Use flags to prevent the light from the slaves hitting the camera directly. If the slaves and the flags then show in your image, you may be able to remove them with Photoshop.
BTW, this is the long ETTL cord I have: