Thanks to Ken! While this would also work for a single monitor system, I just don't see the reason. So, I'm going to embellish Ken's instructions for us people using two monitors...
1. Don't stretch
Blue Iris to two monitors, you'll hate playing clips (try it...)
2. Open Blue Iris full screen in one monitor (Personal preference here, but I prefer the right one)
3. Select the group you want from the top menu (All cameras or whatever. I have a "Home" group that I prefer)
4. Select Edit layout from the top menu
5. Drag a camera that you want over to your 2nd monitor
6. Drag all the other cameras that you want on your 2nd monitor ON TO that first camera. A frame will appear around that group of cameras
7. Drag the cameras in that group into whatever order suits you
8. Deselect Edit layout
9. Maximize that frame (it will only fill your 2nd monitor's screen)
Now you ask, what does that do for me...
1. All clips from any camera play ONLY in the 1st monitor
2. Clicking on a camera in the 1st monitor works just like it always has
3. Double clicking a camera in the 2nd opens that camera full screen in ONLY the 2nd monitor
I hope this is clear...