Please note union will give you distinct rows. If you want to return duplicate rows if they exist in the two tables you can use UNION ALL. If duplicate rows are OK it will save the overhead of making the result set unique.
Declare @T Table
(
Id int identity(1,1),
First_Value Int,
Second_Value Int
)
insert into @T values (55,10),(65,70)
select * from @t
unpivot (
[values] for [type] in (First_Value,Second_Value)
)UPVT