I agree with Strawberry that you should use 5 channel separation but I suggest the main problem is WPA. WPA modifies the preshared key and this modified key is not passed between these access points, hence you would stay with the same access point you first authenticate with until the signal is lost.
There are solutions to overcome this but not using the WRT54GL (e.g. Colubris MSC + mobility). Try using WEP encryption to prove the point, as this should work.