Minimal Repro to show that two source generators can indeed chain on the PostInitialization output of each other (not to be confused with the output of Execute)
To run just do:
dotnet run --project TestApp
What is the basic idea:
GeneratorOne is introducing an attribute Test where classes that have this attribute get a companion partial that can SayHello()
GeneratorTwo produces a class as part of its PostInitialization that uses the attribute and subsequently gets its SayHello method.