hi.
I use a different user to prevent this mistake in live-situations!
this user has limited functions.

the stereos or the bus-setup can only changed by the administrator-user, and i use this profile only during the soundcheck

-sorry for my broken english-

Ouch! []
Changing the bus architecture or creating stereo channels reconfigures the DSP and resets mix settings too. It safely mutes the audio until reconfigured. This is not intended to be done mid show. These are things you should set up before sound check and then leave for the show.

You should always store everything to a Scene before making DSP changes, then recall that Scene to restore those settings afterwards.

As Toby says, what you really needed to do in your case was simply to gang those two channels. That is instant, and gives you the option to leave certain parameters such as channel pan unganged.

Use ganging for on the fly live mixing.
Hope that helps,
Carey