How to wire centre tap transformer
If you have a transformer with two identical 20 volt secondaries, then you can connect them in parallel to get twice the current of a single secondary, or you can connect them in series to get either twice the voltage out or a center-tapped transformer with 20 volts on either side of the center tap.

Here's how to find out what's what:

1) Connect the primary to the 230 volt mains, or to some lower AC voltage if you're not familiar with working around high voltages.

2) Connect one wire from one secondary to one wire of the other secondary.

3) Measure the voltage across the other two ends of the secondaries.

4) If you measure a very low voltage and you connect those ends to each other, the windings will be in parallel, but if you measure about 40 volts, the wires you've already connected will be the center tap of a 20 - 0 - 20 secondary.