首先定义消息常数，这个是A，B公用的

const
CM_SHOWFORM = WM_USER + $1001;
CM_HIDEFORM = WM_USER + $1002;

A:
TMainForm = class(TForm)
procedure CMSHOWFORM(var Msg: TMessage); message CM_SHOWFORM;
procedure CMHIDEFORM(var Msg: TMessage); message CM_HIDEFORM;
end;

{ TMainForm }

procedure TMainForm.CMHIDEFORM(var Msg: TMessage);
begin
Hide;
end;

procedure TMainForm.CMSHOWFORM(var Msg: TMessage);
begin
Show;
end;

B，找到A的FORM的Handle之后发CM_SHOWFORM或者CM_HIDEFORM消息过来就行了