Here's a sample design in 10 HP. I have no idea how to embed images, so a link instead.
It's called the "Stereo ! Mix", pronounced with a "bang", as in negation. That describes the standard use.
- With the fader to the left, stereo A is sent to main.
- With the fader to the right, stereo B is sent to the mains.
- With the fader in between... well, it's a mix of the two.
The headphone output defaults to the "!" on the switch, meaning that the opposite signal is sent to the phones. This allows automatic cueing, without the need for cue buttons. Only the cross-mix fader is used in live performance.
But if you like, you can switch the headphones to monitor the mains. This can be useful in an environment where otherwise the mains are too quiet to hear.
I built a module like this in Reaktor that I've used for years. Here I've added CV control of the three input pots.
Could be smaller than 10 HP, but then it would be cramped. ;-)