Blending data in Tableau Desktop will not add rows for the second set of dates. Instead it will relate those dates to rows and already have dates from the first data set. If the two data set have all the same columns they can be combined into a single data set that has the entire date range.
A good way to do this is to connect to the first file and create a data extract. Then the second file can be added with the Add Data from File… selection from the Data >Extract menu.
We implement Horizontal Merging , When data is located data in Multiple Databases & Have to use joins. ( For An Example DB1 Having 1 Table, Columns are A,B,C,D and DB2 Having 1 Table, Columns are A,X,Y,Z)
We implement Vertical Merging, when Same Kind of data structure available in multiple databases & have to union
( For An Example DB1 Having 2014 year data include 1 Table, Columns are A,B,C,D and DB2 Having 2013 Data including 1 Table, Columns are A,B,C,D)