The reason why there's two warnings is because each side (Capt. and F.O.) can play it independently. I've been recently able to confirm this by talking to a friend of my dad who is a former 767 pilot. It is also demonstrated in this video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-oUbQbQTxuzk.htmlsi=9ot1Ln_ZUANoq5L-
I think that the takeoff config warning of the 757 and 767 has two tones, one for the CAP side and one for the FO side. They can play independendantly from each other and play at a slightly different speed and thus they go out of sync (I believe this is to help identify which side prompted the warning in case of a crash/mishap, similar to how the MD-80 has 2 stall warnings). You can even hear this in the CVR of Aeroperú 603 for example: for most of the CVR, only one side plays the the warning, prompted by the fact that one of the airspeed indicators was erroneously telling the plane it was overspeeding; towards the end of the recording (when the gpws starts issuing the "too low terrain" warning repeatedly) you can hear a second warning come in, I believe this was because both the speed indicators were telling the plane it was overspeeding at that point.
