A CUE file is a standalone file by default. A package clause allows a single configuration to be split across multiple files.
The configuration for a package is defined by the concatenation of all its files, after stripping the package clauses and not considering imports for now.
Duplicate definitions are treated analogously to duplicate definitions within the same file. The order in which files are loaded is undefined, but any order will result in the same outcome, given that order does not matter.
File a.cue
package config foo: 100 bar: int
File b.cue
package config bar: 200
Result
foo: 100 bar: 200