Adding rows from case statement - SQL SERVER : The current results i have are this


I am comparing the totals from the two tables on a monthly basis. When the total is different between the 2 tables i want to add the difference to another column

    | MonthYear | Person | Table1 Amount | Table2 Amount | Unknown |
    |-----------|--------|---------------|---------------|---------|
    |    Jun-17 |    Tom |           100 |           125 |      25 |
    |    Nov-17 |        |            50 |           150 |     100 |
    |    Sep-17 |    Ben |            50 |            50 |       0 |

Which i have achieved but how do i add the case statement as a row instead instead of a column e.g output like this. I can then group via the 'unknown' category.
    
    | MonthYear | Person | Table1 Amount | Table2 Amount |  Difference |
    |-----------|--------|---------------|---------------|    ---------|
    |    Jun-17 |    Tom |           100 |           125 |          25 |
    |    Nov-17 |        |            50 |           150 |         100 |
    |    Sep-17 |    Ben |            50 |            50 |           0 |
    |    Jun-17 |Unknown |            25 |          Null |           0 |
    |    Nov-17 |Unknown |           100 |          Null |           0 |

Can this be done? Any help would be appreciated. Thanks