I've modeled a house in which I later on plan to add some grass around. Since the render looks much better in Mental Ray I prefer this, but Mental Ray doesn't render painteffects...
So I just tried to render layers using some flowers instead of grass to begin with, and I started with just rendering in Maya Software (I'm a real newbie when it comes to rendering).

I rendered the house only - looks great - even the shadows from the flowers appear on the house although the flowers themselves did not render. Perfect! I saved this as a bmp.

I rendered the flowers - looks great - the flowers standing behind the house are almost black from the shadow of the house - which itself wasn't rendered. Perfect! I saved this as a tga.

I opened Photoshop and loaded the two images. But when I put them together it just doesn't work. All of a sudden you can see that the flowers don't belong there - it looks like they've been cut in. I expected a result which looks exactly the same as if I would have rendered the whole image together (the house and the flowers).
What did I do wrong?

/Thanks!