I’m not sure when this was introduced, but in the 4.26 source code there is support for including one .ini from another. It looks like it will only include exactly one file, and the include must be the first line. An included .ini can include another .ini tho. It would not be difficult to modify the source to allow including multiple .ini files, maybe Epic will do that one day.
The source can be found in FConfigFile::Combine
and syntax is a line that starts with #!
immediately followed by a path relative to the current .ini file of the other .ini file to include.
For example:
#!../Windows/WindowsEngine.ini
The source comments say this:
// this will import/"execute" another .ini file before this one - useful for subclassing platforms, like tvOS extending iOS
// the text following the #! is a relative path to another .ini file
// now import the relative path'd file (TVOS would have #!../IOS) recursively