The ones that I use a lot of, that have two pairs of primary leads, are spec'd to run from either 115VAC or 230VAC. They actually have dual primary windings. For 115VAC, you just use the two primary lead pairs in parallel. And for 230 VAC, they are wired in series. That way there is 115VAC across each primary, either way, and the secondary outputs are the same for either 115VAC or 230VAC.

A center-tapped secondary just gives you a "middle" voltage. Remember that "voltage" is (only) defined relative to another point. So it's up to the designer to decide what to do with the center tap (versus the other two). But, for example, it could be defined to be zero volts, for a dual-polarity power supply. (But someone could just as easily define one of the ends of the secondary to be zero volts, and the other two could then be two positive or negative voltages, with one twice the magnitude of the other.)

A transformer listed as 24-0-24 gives 48 VAC RMS (or about sqrt(2) times that, zero-to-peak, i.e. 67.9 v pk), between the two outer secondary leads, and 24VAC between each outer secondary lead and the center tap lead. For a dual mono setup, you'd probably want both positive and negative voltage rails for each mono block. So, unless they were single-supply designs, you'd need all three secondary wires for each one. (That should be able to work. But I think I'd just use two lower-power tansformers, for that, if I could.)

I usually would rather have two center-tap leads, i.e. dual secondary windings, since some types of dual supply designs will not work with just one center tapped secondary (e.g. using two positive voltage regulators to get a dual-polarity supply).

If the transformer has the dual primaries and also has dual secondaries, i.e. four leads for each (eight leads total), then you have the most options: The secondaries can be used in series, for twice the voltage with half the max current (compared to parallel). Or they can be used in parallel for half the voltage with twice the max current (compared to series). They can also be used independently, for two separate power supplies with either opposite or same polarities. And the primaries can be wired for either 115VAC or 230 VAC without affecting the secondaries' voltage or current ratings.

By the way, here's a link to a site that has a clear explanation of the calculations needed, for simple transformer-based power supplies:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm