В принципе, самый простой способ - сделать ваш союз производной таблицей или CTE, а затем сгруппировать их по дате.
IE.
select
sum(dt.ViewsThatMonth) as ViewsThatMonth
,dt.[day]
,dt.[month]
,dt.[year]
from
(select Count(emailID) as ViewsThatMonth, Day(entry_date) as day, Month(entry_date) as month, Year(entry_date) as year from email_views
where emailID = 110197
Group By Day(entry_date), Month(entry_date), Year(entry_date)
UNION ALL
select Count(Record_Entry) as ViewsThatMonth, Day(Record_Entry) as day, Month(Record_Entry) as month, Year(Record_Entry) as year from dbo.tblOnlineEmail_Views
where emailID = 110197
Group By Day(Record_Entry), Month(Record_Entry), Year(Record_Entry)
) dt
group by [day], [month], [year]
order by dt.[year], dt.[month], dt.[day]