create table placeh (
   id int not null auto_increment primary key
  , one int
  , tw int
);

create trigger bu_placeh before update on placeh /* table name, NOT column name! */
for each row
set NEW.tw = OLD.one; /* for one-statement trigger body BEGIN END not needed */

insert into placeh(id, one, tw) 
values(1,11,210),(2,12,220),(3,13,230);

select * from placeh;
| id | one | tw  |
|----|-----|-----|
|  1 |  11 | 210 |
|  2 |  12 | 220 |
|  3 |  13 | 230 |

update placeh set one = one * 2 where id > 1;
select * from placeh;
| id | one |  tw |
|----|-----|-----|
|  1 |  11 | 210 |
|  2 |  24 |  12 |
|  3 |  26 |  13 |