I haven't tried to use the range extender mode, so I'm not sure if there's a specific reason why. Here's my experience with wired AP's on the same SSID:
I've set up multiple (WIRED) AP's before on the same name. Make sure you broadcast the secondary network on a different channel than the other network. (Use 1,6,or11 only as a best practice, as they're the only channels that don't overlap.)