So. Does the DoR tileset have mountains with a grassy top and a sand bottom?
DoR set help (...or error?)
Depends. If your mountains are more than one tile tall, just use the tops from the grass set, and the bottoms from the sand set.
As Russadwan said, if it's 1 tile big(seems unwise to use that) you need to take the bottom part(8 pixels) of the sandy bottom and paste it over a grassy top(you do that by copying it and pressing "o" on the grassy top).
It's as easy as that.