set nocount on;

declare @t table( EmployeeID int, DateStart datetime, DateEnd datetime);

insert @t
  select 1, '20190101 10:07', '20190101 20:07'
  union all
  select 1, '20190102 10:07', '20190102 20:07'
  union all
  select 2, '20190102 15:07', '20190102 20:07'
  union all
  select 3, '20190102 15:07', '20190102 18:07'
  union all
  select 1, '20190103 10:07', '20190103 20:07'
  union all
  select 2, '20190103 14:07', '20190103 20:07'
  union all
  select 3, '20190103 15:07', '20190103 18:07'
  union all
  select 3, '20190103 18:10', '20190103 19:07'

-- начала/концы наложений
declare @b table( Date datetime primary key, n int identity unique );
insert @b( Date)
  select DateStart 
  from @t as t where exists( select * from @t as x where x.EmployeeID <> t.EmployeeID and t.DateStart between x.DateStart and x.DateEnd )
  union
  select DateEnd
    from @t as t where exists( select * from @t as x where x.EmployeeID <> t.EmployeeID and t.DateEnd between x.DateStart and x.DateEnd )
  order by DateStart
;
select * from @b;

-- интервалы перекрытий
with t as ( select d1 = b.Date, d2 = e.Date
                 , dT = datediff(minute, b.Date, e.Date) from @b as b inner join @b as e on e.n = b.n + 1 )
 select * from t
   where exists( select * from @t as x where dateadd( minute, datediff(minute, d1, d2), d1) between x.DateStart and x.DateEnd )
 ;

-- сумма интервалов перекрытий
with t as ( select d1 = b.Date, d2 = e.Date
                 , dT = datediff(minute, b.Date, e.Date) from @b as b inner join @b as e on e.n = b.n + 1 )
 select minutes = sum(dT) from t
   where exists( select * from @t as x where dateadd( minute, datediff(minute, d1, d2) / 2, d1) between x.DateStart and x.DateEnd )
 ;