A center tapped transformer cannot be wired for double the current at half the voltage. A transformer with 2 windings (4 wires) can. If your transformer is like that, you could have a switch to have 2 ranges of voltage and you would save power on any voltage < 18V when using the lower range.
If it is really centered tapped, you can still do the same, but you will not double the current output.
Bob
If it is really centered tapped, you can still do the same, but you will not double the current output.
Bob