Can 2 organisms from the same genus be from different phyla? Why or why not?

No.

Explanation: The classification system from most general to most specific is kingdom, phylum, class, order, family, genus, species. Things in the same specific category can't be in different general categories.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.