Můžete provést pouze podmíněnou agregaci:
select table_name,
max(case when date = '2016-09-14' then round(((data_length + index_length) / 1024 / 1024), 2) end) as size_20160915,
max(case when date = '2016-09-15' then round(((data_length + index_length) / 1024 / 1024), 2) end) as size_20160916,
(max(case when date = '2016-09-15' then round(((data_length + index_length) / 1024 / 1024), 2) end) -
max(case when date = '2016-09-14' then round(((data_length + index_length) / 1024 / 1024), 2) end)
) as diff
from DBA_DB.table_growth_history t
where date in ('2016-09-14', '2016-09-15')
group by table_name;