create table table1(id char(1), val1 integer, val2 integer);
insert into table1 values ('A', 0, 0);

update table1
   set val1 = val1 + 1,
       val2 = val1 + 2;

merge into table1 t1
using (
        select 'A' as id, 1 as val1 from rdb$database
        union all
        select 'A' as id, 2 as val1 from rdb$database
      ) t2
  on t1.id = t2.id
  when matched then update
  set t1.val1 = t1.val1 + t2.val1;