Two relations to same table in SQL Server
It is not possible with a constraint. Have a look at Multiple Cascading Actions. 


  No table can appear more than one time in the list of all cascading
  referential actions that result from the DELETE or UPDATE.


You can use an instead of trigger instead where you do the update in Table2 before the delete in Table1.

The trigger could look something like this

create trigger Table1_Delete on Table1 instead of delete
as

update Table2
set Table1ID1 = null
where Table2.Table1ID1 in (select D.Table1ID from deleted as D);

update Table2
set Table1ID2 = null
where Table2.Table1ID2 in (select D.Table1ID from deleted as D);

delete from Table1 
where Table1.Table1ID in (select D.Table1ID from deleted as D);