Your 12v - 0 - 12v 2.5A - can be used as a 24v 2.5A transformer ... you simple leave the center tap pin/wire disconnected, unused.
Some transformers have a center tap which can be separated, to create two separate secondary windings. In this configuration, you can wire the secondary windings in parallel to get double the current, but half the voltage - so you get a transformer that outputs 12v AC at 5A.
If the center tap can not be separated, you can still get higher current but low voltage but not using a bridge rectifier, see below :
(i'll edit this message further in a few minutes)
