WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
You'll need to redesign the PSU for double the current at each winding. That means a transformer with double the current rating, using rectifiers with double the current capacity, doubling up the warmup control 5AR4, and modifying the filtering (bigger C, bigger L, more current capacity for the L) to drop the ripple back to the single PSU values.
It's not trivial, but it could be done. Given how exquisitely sensitive SET is to the power supply, you want to do this carefully and systematically to avoid burning up money needlessly.
The referred circuit looks a Japanese one and I notice particular power transformer which was once popular in the market. The high voltage windings have enogh current capacity to support two channel amplifiers. Also, 5AR4 and choke coil can work for two channnels. Thus what you additionally need is another filament supply (10.5V) which is not shown in the drawings.
I build a similar amplifier more than 20 years ago and it works pretty fine up to now.