Encode two R11G11B10F buffers into one using Y'CbCr? We are trying to optimize Lighting in fillrate. Integrated graphics chips are very slow in memory access.

There are two Light Buffers: one for Diffuse and one for Specular. Both use format R11G11B10F. Is there any way to pack two buffers into one? For instance, using color model Y'CbCr:
You might want to look at The Compact YCoCg Framebuffer. It uses a 2-channel buffer to store luminance for every pixel and the two chroma components in half the pixels each, forming a checkerboard. It also uses an edge-aware upsampling filter at the end of the frame to reconstruct the missing chroma components and convert back to RGB.

You could extend this to a single 3-channel buffer to store the diffuse luminance, specular luminance, and checkerboarded chroma (assuming based on the diagram in your question that it suffices to store just one pair of chroma values).

You might also consider using R10G10B10A2 format instead, unless you really need the range of the floating-point format. It's the same amount of bandwidth but will give you a good deal more precision (R11G11B10F only gives you 5–6 bits of mantissa per component). Plus, you can use the 2 bits of alpha to store a couple of flags, such as your diffuse/specular flag.