I have had the WZR-HP-AG300H for a while, and I was able to use both the wireless interfaces without any problems.
Some time ago, perhaps after I have upgraded to 19154 firmware, the connection through ath1 stopped working for me. Unfortunately, I did not really pay attention to this, and I cannot tell when it happened.
I have tried all combination of settings. I also went ahead and run "erase nvram" using telnet in hopes that there was some pesky setting somewhere that was not reacting to my resets. Nothing helps. I have two macs, one with Lion and one with Mountain Lion. Both give me "A connection timeout occurred" error when trying to connect to ath1 (ath0 has no problems). If I turn on WPA2, Lion asks me for the password first, but Mountain Lion goes straight to the error. I am at a loss. I tried everything I could. I hope that someone here will have some advice. I am attaching some screenshots with my current configuration, but I have tried many more (including mixed mode, 20MHz channel width, no security, etc.).