IMHO, if you have the space in your mash tun, it would be better to put more of the water in the mash.
In my experience not a lot is gained by a double batch sparge over a single, as long as the sparge water is held for 15 minutes or so and I stir. I do double batch sparges when I have to because my mash tun is small.
Again, this is just my opinion, but it seems like you are putting a lot of sparge water through. It could be getting really thin by the second sparge.