Why parallel diodes on the output of a switch-mode?
The parallel Schottky diodes are there to allow a higher forward current through the load. Usually done to prevent scenarios where the load current exceeds the maximum current rating of a single diode.

https://www.daenotes.com/electronics/basic-electronics/diode-in-parallel

EDIT: You can refer to the datasheet, page 23 for a similar design example. The diode packaging is shipped as a single component (MBRD1035). http://www.ti.com/lit/ds/symlink/lm25118-q1.pdf