If both are flashed with DD-WRT it might work.
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I'd think that only the WRT54GL would really need the DD-WRT firmware to act as a repeater, and he could keep the WRT610N on the stock firmware.
I found some instructions here:
http://www.dd-wrt.com/phpBB2/viewtopic.php?p=214186&sid=6a3c9bb0ebe4803a0451fe9860ed1506
I'll bet that he could pull off the same stunt with the Tomato firmware by setting it to WDS mode, but I can't seem to find a walk-thru for that setup.