Sampling DRM_FORMAT_YUYV in GLSL

[Volker Vogelhuber]

I'm currently trying to render an V4L2 image with OpenGL
on an Intel Apollo Lake using Linux 4.10 and Mesa 13.
Supprisingly I noticed that importing a DMABUF with format
DRM_FORMAT_YUYV into OpenGL using
  eglCreateImage/glEGLImageTargetTexture2DOES
worked out of the box. But I noticed that there seem to be a split into
two textures when importing the buffer. At least the nplanes
variable in the __DRIimage's planar_format is set to 2 and in the
intel_screen.c there is a comment for __DRI_IMAGE_FOURCC_YUYV:
"For YUYV buffers, we set up two overlapping DRI images
  and treat them as planar buffers in the compositors."
So far so good, but how do I access the second texture in the GLSL
shader. I only created one texture object before calling
glEGLImageTargetTexture2DOES with the EGLImage I got from
eglCreateImage. And using texture( source, texCoord ) on this
texture samples only the Y and U channel, what seems obvious
as the texture is created as GL_RG texture.
Can anyone explain to me, how one accesses the second plane.
And if there is an example somewhere how the two planes
have to be sampled to convert the values back to RGB, it would
be nice if someone can send a link to such an example.

Thanks