Won't this work?

public IEnumerable<(Class1?, Class2?)> DoSmth()
{
   foreach (...)
   {
      var output = DoComplexCalc();
      
      if (output.Whatever)
      {
         yield return (new Class1(output), null);
      }
      else
      {
         yield return (null, new Class2(output));
      }
   }
}