Flattening sprites into one texture

Hi, I would like to know how can achieve the following effect:

I have a blue square bitmap, and I want to do some shapes (like tetris shapes).
I’m doing several CCSprites with opacity at 50, so in the end I have the shape I want. But there is a problem, I can see a one pixel line in the edges of each shape; this is not strange i guess, because the opacity is set to the individual sprites, and I also guess I can see that overlapping line because the aliasing.

So, what I want to do is to take those groups of sprites forming the shape, and “flatten” them, so I can have one sprite. To this sprite I think I should be able to set opacity without any glitches. Am I right?

I was looking certain things like CCRenderTexture and CCTexture2D, but I don’t know how should I use those classes.

Thanks in advance!