Noob to Pro skybox tut question


In case the answer to my last question turned out to be a skybox or skydome I was trying the tut in the Noob to Pro wiki. I have attached a test render after creating and texturing the domes. There are two white lines - the one at the horizon is explained by the low quality texture but there is a thin line above the horizon where the edge of the lower dome is. Does anyone know how to remove this?

Here is the attachment.