SQL update multiple rows in destination table with same id but different values from source table
Couldn't find a SQL Server 2008 fiddle engine so I had to opt for a SQL Server 2014 ... so not sure if the following will work in SQL Server 2008, but fwiw ...

Setup some sample data:

create table Table1(id int, Date datetime null);
create table Table2(id int, Date datetime);

insert Table1 values (1,null)
insert Table1 values (1,null)
insert Table1 values (2,null)
insert Table1 values (2,null)
insert Table1 values (2,null);

insert Table2 values (1,'2013-01-29 08:50:00.000')
insert Table2 values (1,'2013-01-29 15:28:00.000')
insert Table2 values (2,'2013-01-31 11:56:00.000')
insert Table2 values (2,'2013-03-11 16:08:00.000')
insert Table2 values (2,'2013-01-31 14:04:00.000');


Keeping in mind that we haven't been provided (yet) with any means to determine which rows to match between Table1 and Table2 for a given id value, I'll just let row_number() generate a 'matching' rowid.

And then we'll make use of SQL Server's ability to update Table1 via a derived table definition:

update T1 
set    T1.Date=T2.Date

from   (select row_number() over(partition by id order by Date) as rowid,
               id,
               Date
        from   Table1 
        where  Date is NULL) T1

join   (select row_number() over(partition by id order by Date) as rowid,
               id,
               Date
        from   Table2) T2

on      T1.id    = T2.id
and     T1.rowid = T2.rowid;


And the results:

select * from Table1;

id  Date
--- --------------------
1   2013-01-29T08:50:00Z
1   2013-01-29T15:28:00Z
2   2013-01-31T11:56:00Z
2   2013-01-31T14:04:00Z
2   2013-03-11T16:08:00Z


And here's a SQL Fiddle for the above.