Drawing CCRenderTexture on CCRenderTexture

Hi,

I am trying to draw CCRenderTexture on CCRenderTexture for double buffering.
However, I found some image issue in the following test case:

We have 2 CCRenderTexture, RT_A and RT_B.
Frame 1: Draw images on RT_A, draw RT_A on RT_B
Frame 2: Draw RT_B on RT_A, draw RT_A on RT_B
Frame 3: Draw RT_B on RT_A, draw RT_A on RT_B

After 5 iteration:

After 50 iteration:

I have tried for different pixel format, but getting same result.
Is there any settings I am missing?
Thanks!