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But, seriously, what you would normally do is put the N600 in AP mode, and disable DHCP on one of the routers. However, it doesn’t look like the N600 can be put into a true AP mode.
This guy says you can sorta kinda do it, though:
http://damien.co/general/using-netgear-n600-dgnd3700v1-extend-home-network-8747
Thanks, I might try that on a day when I’ve had some sleep and the wife and kids aren’t home. It really doesn’t look that hard.