Hope this works:
select a.Month, a.TotalCountSub, b.TotalCountCust from (
SELECT Datepart(mm,starttime) as Month, COUNT(*) TotalCountSub
from data
where usertype='subscriber'
group by Datepart(mm,starttime)
having count(*) > 1
) a
INNER JOIN (
SELECT Datepart(mm,starttime) as Month, COUNT(*) TotalCountCust
from data
where usertype='Customer'
group by Datepart(mm,starttime)
having count(*) > 1
) b
ON a.Month = b.Month
order by a.Month ASC