set nocount on
declare @yourTable table (ID int, DATESTAMP datetime, DURATION int, DESCRIPTION varchar(30), CHANNEL int)
insert @yourTable
select 1, '2008-11-28 12:42:25.000', 27, 'Document delivered.', 7 union all
select 2, '2008-11-28 12:46:12.000', 14, 'Document delivered.', 7 union all
select 3, '2008-11-28 12:47:52.000', 15, 'Document delivered.', 7 union all
select 4, '2008-11-28 12:49:32.000', 14, 'Document delivered.', 7 union all
select 5, '2008-11-28 12:49:33.000', 15, 'Document delivered.', 8 union all
select 6, '2008-11-28 12:53:13.000', 15, 'Document delivered.', 6


declare @start datetime
       ,@numDays int
select @start = '2008-11-28'
       ,@numDays = 14

;with cte
as
(
select id
       ,datestamp st
       ,dateadd(second, duration, datestamp) ed
       ,channel
from @yourTable
)
select dateadd(day, datediff(day, 0, a.st), 0) dy
       ,dateadd(hour, datediff(hour, 0, a.st), 0) hr
       ,count(*) * 2 [simul_count]
from cte a
inner join cte b
       on b.st >= a.st
       and b.st <= a.ed 
       and b.channel != a.channel
where  a.st >= @start
and    a.st <= dateadd(day, @numDays, @start)
group by dateadd(day, datediff(day, 0, a.st), 0)
       ,dateadd(hour, datediff(hour, 0, a.st), 0)


output:
dy                      hr                      simul_count
----------------------- ----------------------- -----------
2008-11-28 00:00:00.000 2008-11-28 12:00:00.000 2