Is there a way to have a viewport on external monitor linked to mix 2 whilst mix 1 is going to the stage screen playing something. Prepare the content on mix 2 before switching mix 1 and 2 around so now mix 2 is on stage screen and mix 1 is on viewport on external monitor. the switch between mix 1 and 2 would need to be smooth or snap so like fade time.
What i would do is have 3 mixes, mix one and mix 2 as your main mixes, you can patch these to your preview monitor, and then have your third mix with 2 layers and use this as a relay mix. so lyaer one, will be a relay of mix one and layer 2 will be a relay of mix 2 you can then use a midi controller or pin bridge to make sure you get a nice smooth fade.