workflow : 2 panels to see changes from 1 effect

Hi,

I'm currently learning Keylight, and it seems it's possible to open 2 composition windows side by side, and to see 1 view in the 1st one, and another view in the 2nd one :
for instance, it should be possible to choose , from the View Menu of the effect :
- "Source" for the 1st comp. windows, and
- "Screen Matte" for the 2nd one.

There's no problem to open 2 windows side by side, but when I change the View in the View Menu of the effect, both windows change, and there is no way (so far) to compare one view and another side by side ... :-/

Do anybody know how to do it ?

Thank you ! :-)