You may need two transformers
First transformer secondary you put a label A and B... second transformer secondary wires you put label C and D.
Now join B plus C... this gonna be your center tap
Now you have three wires.... A, B plus C and D
Your transformer AC output voltage must be aproximately 23 volts output..then you gonna have 23 plus 23.... after rectification and filtering you gonna have 64 plus 64 Dc volts.
About the power of each one of these transformers.... well...this you must decide depending your case... as you can use one single transformer to feed two channels, or one transformer to feed each channel (your amplifier, the stereo unit, will have 4 transformers in this case).
You take a look in the power specifications, already published in this thread, and study your own specific case..your speaker power...your speaker impedance.....then you must decide what is reasonable to your home, and all this depends on your decision and depends on your speakers...then, you will conclude the transformer power (VA ratio that means the same as Watts)
Do not use any other trick to "create" or "simulate" a dual supply using a transformer that has only two wires in the output (secondary)... this never works fine.... do not try.
Or you buy, or build a transformer with three wires in the secondary or you use two power transformers to "create" a single unit.
If you need a transformer of 800 watts..then each one of your dual wired secondary transformers must be a 400 watts unit.
regards,
Carlos