Mounting to hollow wall through slate tile
You could use a toggle for one of the holes and a very small masonry screw for the other.  The toggle will be doing all the heavy lifting, the small masonry screw will just be there for looks and to keep the robe hook from rotating.  

Spax and other brands make small screws that can be used for light loads in masonry without an anchor.  The smaller sizes require only a 1/8" or 5/32" pilot hole.  That would give you about a quarter inch between holes.