Seriously, until you really know better. Delete all terrains but one and work with that. That's a mess.
I recommend even if you want to stick just with sand and grass, get more paint textures (more sand and grass options) and mess around, cover the margins, where are meeting 2 different textures, with rubble (i like the TR_rock_slate_rubble_B a lot because its partly transparent / with alpha) and or later of course with instance painter (which does a great job and covers up all kind of texture paint ugliness).
After layouting the Terrain paint, i recommend:
paint with the smallest size brush
make use of right click delete (see it as a paint function for the texture down aswell)
some examples (also see the amount to textures):