You are not the first soul looking for that schematic.
As far as I know, it has never surfaced.
The Twin 12 had two output transformers, didn't it?
This Silvertone 1474 may help with that:
Here is the 1396. The preamp tubes are 12AX7
Here is a link of other Silvertone amps.
You may find a preamp that is similar.
Silvertone Schematics - Tube amp Schematics