Ok, so I didn't try very hard but I did try it, nonetheless.
I couldn't be bothered looking through all my cameras to find a suitable viewfinder so I just used my Centon SLR (and my Sony Alpha). I didn't even remove the yellow filter, how lazy is that?
But, really, I was just trying it out to see what the effect would be and this is what I got....
The difficult part (which I clearly didn't master) was trying to ensure both cameras were focused correctly!!!