Why? The 2” eyepiece holder.
The GSO Dielectric Diagonal optical path:
1. 12mm SCT visual back.
2. 52mm diagonal.
3. 40mm removable eyepiece holder.
This 104mm diagonal plus a 110mm binoviewer will create a 214mm optical path.
How to quickly swap between 2” eyepieces and a binoviewer:
Unscrew the 2” eyepiece holder, fit it with an interior ring to secure the 1.25” binoviewer nosepiece tightly. Whittle, file, hacksaw and glue a 10mm binoviewer/diagonal adapter together with threads on both ends.
Now you have either a standard 2” diagonal with the purchase of a new threaded eyepiece holder OR a binoviewer and 2” diagonal adapter with a 184mm optical path.