Hey folks...

is a 64 kbps mono MP3 file as good as an 128 kbps dual channel MP3? What about stereo / joint stereo - I guess that doesn't apply there for sure since bits can be shared across channels, or am I wrong?

Sebastian

2 mono files will have 2 times more headers than 1 dual channel file. So the dual channel file will have 4 bytes extra for audio data per frame.

It's actually 5 bytes, as a 64kbps frame is 208 bytes and a 128kbps frame is 417 bytes.

Basically "dual channel" means that you have to streams of independant content. (exemple: dual language)
While you'd better not use any interchannel masking, and obvisouly it's forbidden to use joint stereo, you can still distribute bits based on channels complexity.
You can consider it as a constant bitrate multiplexing of 2 vbr channels.