You cannot stitch only 2 images from a Canon 1Ds or 5D with a lens that is only 180 degrees as there is no overlap. To be actual the lens isn't even 180 degrees when used on these cameras it is just under. No overlap = no stitch.

This was not an issue with the Nikon Coolpix and FC-E8 and FC-E9 as both lenses were 183 degrees (actual) and on some models up to 190 degrees.

3 shots is more than enough overlap and the results are great.

Regards, Smooth

As zxcvbnm has suggested tilting up will help. Tilt up only about 5 to 10 degrees. This will help clear some of the black edging but will increase the hole at the tripod (bottom) slightly. Really you should be editing this out using Photoshop, it is only a 3 to 5 minute job. The other alternative is to stitch user software that supports the use of a blender like Smartblend or Enblend. Hopefully the next version of Panoweaver will support these plug-ins.

Also, shooting RAW and processing the image correctly would also help with this matter as you have control over the Vignetting (light fall off) towards the edges of the lens. You could also try enclosing the circle within Panoweaver to trim slightly the edge area of the fisheye image circle. 

Regards, Smooth