You cannot do this in Java.
As a note, if you were using a language that supports partial classes (such as C#) you could do
File 1:
partial class OuterClass {
}
File 2:
partial class OuterClass {
class StaticNestedClass {
}
}
but Java doesn't have partial classes so this wouldn't be of help for this particular situation. In Java a class can only be declared in one file.