Assembler MC: When is combine button available?

Hi,

I run MC at nominal and one more corner (T=125C), but I want the histograms for nom and this 2nd corner in one plot. I remember this is possible, but why is the button "Combine" grayed out?

I also wonder about plotting template, can I switch combine option (when it works) afterwards, or do I need 2 plotting template to get with one separated histos, and in the other combine?

Bye Stephan