Tiles Offset

AFAIK with Tilemaps the offset of the tiles is based on the size of the tile. Is it ever going to be possible to get access to the offset rather than having the offset the size of the tile ? The main reason I want to do this is I would like to have tiles 20 units by 40 units but have the offset as 20 units by 20 units. This would allow so really neat perception benefits. I know I could make another layer and split the tiles but this does become painfull and chews up a layer.