If it's with netstumbler its another network. Look at the chanel, you may want to move yours to 1, 6, or 11. Further the better.
The WRT54 is only capable of broadcasting one SSID. USR makes a 5453 AP that has the capability of operating 2 SSID from 1 AP, I use it's little brother 5450 AP. One great unit, with extended range.
As you can see netstumber is capable of capturing SSID Hidden or Not. If there is any traffic to a hidden SSID it will expose it self. So make sure you use the max length for WPA. GRC.com has a random key generator that is real good. You will have to use copy and paste with it. It is what I use for Password & Key's.