unit uEgg;

interface

uses
  uChicken;

type
  TEgg = class
  private
    FChicken: TChicken;
  published
    property Chicken: TChicken read FChicken write FChicken;
  end;

implementation
. . .
end.

unit uChicken;

interface

uses
  uEgg;

type
  TChicken = class
  private
    FEgg: TEgg;
  published
    property Egg: TEgg read FEgg write FEgg;
  end;

implementation
. . .
end.