I'm on a fresh Debian Buster installation with packages
fonts-symbola. I want to use "Noto Sans Mono" as the main font for XTerm, so I set the following line with
xterm*faceName: Noto Sans Mono
That works. Now, if a symbol isn't found in "Noto Sans Mono" (🔒 for example), XTerm automatically uses the one from "Symbola Regular". However, I want XTerm to look for the symbol in "Noto Sans Symbols2 Regular", first - and only if that font also doesn't contain the symbol (🔑 for example), then look in "Symbola Regular".