Why (as in the case of Matot/Maasei) are two Torah portions read on the same Shabbat?

There are more Torah portions than there are Saturdays to read them on, in most years. As a result, portions are often combined. There is a formula for working it out based on the fact that certain portions should be read by certain dates on the Hebrew calendar.

In the case of Matot/Masei, they are combined this year in order the portion of Devorim should be read prior to Tisha B’av, and the portion of V’Etchanan immediately afterwards. The reason why we desire that is as Devorim is full of rebuke, we wish to have the rebuke behind us by the time Tisha B’Av has passed. As V’Etchanan contains the second reading of the Ten Commandments we wish to read it right after Tisha B’Av in order to renew our acceptance of the Torah after such a calamity.