Lots and lots of ways to approach this.
Try this:
1. Create a bus called 2Bus. Route all of your tracks/buses whatever here. This is your master bus used for mixing (bus plugins, etc.).
2. Create a bus called MAINS and route this to the physical outs of your primary monitors.
3. Route the out of the 2Bus to MAINS.
4. Create a bus called BEHRITONE. This should send to the physical outs for your Behritone.
5. Create a Send on the 2Bus and route it to the BEHRITONE.
4. Mute the BEHRITONE bus.
5. Create a group for the Mute button on the BEHRITONE bus.
6. Add the Mute button for the MAINS to the group.
Now you have two buses that route to two different physical outs, and the mute buttons toggle each other so only one is ever active at once.
I hope this helps! :)