Parece que você está procurando agregação condicional:
select
u.user_id,
u.username,
min(case when a.action = 'IN' then a.dateTime end) `in`,
max(case when a.action = 'OUT' then a.dateTime end) `out`
from
user u
inner join user_attendance a on u.user_id = a.user_id
group by
u.user_id,
u.username,
date(a.dateTime)
order by
u.user_id,
date(a.dateTime)
Demonstração no DB Fiddle :
| user_id | username | in | out |
| ------- | -------- | ------------------- | ------------------- |
| 1 | tiger | 2019-10-01 08:00:00 | 2019-10-01 14:00:00 |
| 1 | tiger | 2019-10-02 08:00:00 | 2019-10-02 14:00:00 |
| 2 | pooh | 2019-10-01 08:00:00 | 2019-10-01 14:00:00 |
| 2 | pooh | 2019-10-02 08:00:00 | 2019-10-02 14:00:00 |
