The TSOAG9T2 appears to have a male T2 thread for the guide camera. The SBIG ST-i has a female 28mm (1.25" filter) thread. There are a few ways to get there:
1. T2 to M28 adapter. Something like
this or
this should work - but you should double check with the sellers as I can't tell from the listings.
2. You could also use a
T2 to C-mount adapter combined with a
C-mount to 28mm adapter to attach your ST-i.
Note that you may potentially need a spacer for either your main camera (M48 mount) or your guide camera (T2 mount) depending upon how far away the respective focal planes are.
By imaging camera, I assume you mean your Canon 6D? It sounds like you just need an
M48 -> Canon EOS adapter.