1. Use union all:

select emp_no,max(lut) from (
select emp_no,lut from emp1
union all
select emp_no,lut from emp2
)
group by emp_no;

2. Use max in the inline view:

select emp_no,max(lut) from (
select emp_no,max(lut) lut from emp1
group by emp_no
union all
select emp_no,max(lut) lut from emp2
group by emp_no
)
group by emp_no;

3. Use a join:

select emp_no,greatest(max1,max2)
from (
select e1.emp_no, max(e1.lut) max1, max(e2.lut) max2
from emp1 e1 join emp2 e2 on (e1.emp_no = e2.emp_no)
group by e1.emp_no
)