; with
cte as
(
	select	t1.ID, t1.Content
	from	table1 t1

	union all

	select	ID	= case 	when t1.ID is null then t2.ID
				else NULL
				end,
		t2.Content
	from	table2 t2
		left join table1 t1	on 	t2.ID	= t1.ID
)
select	ID = case when 	ID is null 
		  then	max(ID) over()
			+ row_number() over (partition by ID order by Content)
		  else  ID
		  end,
	Content
from	cte
order by ID