The Starizona 0.63x reducer has an image circle of 27mm. The guide sensor that is in your camera is outside of this image circle. To use the guide sensor for autoguiding, an OTA needs an image circle of 44mm to get sharp stars across the guide camera's sensor plane.
The Starizona reducer (barely) corrects for the IMX571 sensor (28mm diagonal), but as you observed, has horrible coma by the time it reaches the guide sensor. Two corners of the guide sensor in the 2600 Duo are barely inside a 44mm circle.
https://starizona.com/products/starizona-sct-corrector-63x-reducer-coma-corrector
https://www.zwoastro.com/product/asi2600mc-duo/
I have the same problem when I use it with Hyperstar
The 11" Hyperstar has an image circle of 35mm.
The now-expired patent (US 5,525,793, granted in 1996, expired in 2014) of a dual camera was awarded to SBIG (now part of Diffraction Limited). For good reasons, SBIG only produced two cameras (the ST-7 in 1994, and the ST-8 after) before abandoning the scheme.
http://www.company7.com/sbig/products/st7.html
https://patentimages.storage.googleapis.com/2b/3d/1f/41b03c5384d960/US5525793.pdf
The sensors in the ST-7 were small, with the center of the guide sensor only 6mm from the center of the main sensor, requiring an image circle of only 16mm. ZWO should have warned potential customers of this, and other pitfalls of a dual sensor camera.
(Notice that even an OAG is covered by this patent -- see Figure 5 and Claim 3.)
Caveat emptor.
Chen
