The amount of color contribution from the sprite image is blended with the color in the background pixel so that the resulting background pixel is some result of the two combining, as a float value. It should then be perfectly possible to draw another sprite at the same coord, next to it, and it should also contribute to that pixel value, which means it also takes into account the other sprite's pixel. You shouldn't need anything else to make them appear to be perfectly lined up and antialiazed with each other.
But it the two are supposed to be joined, you don't want *any* background value being used, jsut the values from the two tiles.