Re: back to back transformers
In theory, the voltage will be increased by 12/10 times so you would get 276V out. In reality, it will be somewhat less, especially uder load becasue you have the losses of two transformers instead of one in the path.
You might consider using an auto-transformer if it's safe to do so in your application or, accepting some losses, you can use a 230V to 20V transformr with it's primary across the 230V and secondary in series with it.
Brian.