SQL & PL/SQL

is it possible to avoid multiple values specification for one bind variable, ie. to specify only one 'X' after using clause in the example below? if so, how should it be written properly?

declare
  s varchar2(100);
begin
  s := ' select rowid from dual where dummy = :1 ' ||
       ' union all ' ||
       ' select rowid from dual where dummy = :1 ';
  execute immediate s using 'X', 'X';
end;

thank you