select to_char(rownum), name, surname from table1 where ...
union all
select '-', '-', '-' from dual
union all
select to_char(rownum + 
(select count(*) from table1 where ...)
), name, surname from table2 where ...

select rownum, name, surname
from ( select 1 tabnb, name, surname from tablea where ... 
       union all
       select 1.5, '-', '-', '-' from dual
       union all
       select 2, name, surname from tableb where ... 
       order by 1 )
/

SQL> select decode(ename,
  2                '-',to_number(null),
  3                sum(decode(ename,'-',0,1))
  4                  over(order by ord rows between unbounded preceding and current row)
  5               ) rn, 
  6         ename
  7  from ( select 1 ord, ename from emp where rownum<=3
  8         union all
  9         select 1.5, '-' from dual
 10         union all 
 11         select 2, ename from emp where rownum<=5
 12       )
 13  order by ord
 14  /
        RN ENAME
---------- ----------
         1 SMITH
         2 ALLEN
         3 WARD
           -
         4 SMITH
         5 ALLEN
         6 WARD
         7 JONES
         8 MARTIN