Actually, two 24" displays are really big, especially if they are placed next to each other and providing you with a screen width of roughly 3840 pixels. And, honestly, there is no use for it, except you are doing some serious stuff like programming, video editing or hardcore browsing on your machine.
But there is nothing like an instant productivity boost. For me, it was the opposite: working around the big screen. So I invented a few rules to utilize the screen estate as useful as possible, and to minimize time needed to rearrange windows ( a tedious task one two screens ) and find lost ones. This is not a universal, scientific-proved guide on how to be more productive with two displays, it's just my experience!
Step 1: Drop the idea of a primary screen.
A primary display is the display that is used to display the task bar or the dock, menu bars ( OS X ) and so on. It's necessary unless your hardware automatically stretches your display across all of your displays, which is not the case here. A primary display is kind of a technical necessity, yet it's further use is quite ... limited.
In case you find yourself preferring one of your displays over the other, rethink your setup. Place your chair so that both screens take up equal portions of your view. And, of course, now is just the right time to minimize any offsets preventing you from treating them as one screen. Books placed under the screen etc. are handy :-)
2. Use both screens for all of your tasks
Just try to do all your tasks randomly on both displays. By doing so for a few days, I just managed to eventually use what i have, in terms of screen estate.
So that's the general part, applicable to all dual-head set-ups. But now for the work. This is just how I did it, once again. There are several approaches, and I use them as I need and like.
1. Approach: Logical Split
I named this because whenever I use this, mostly for general tasks such as reading mail and surfing, I place my browser in the right display, while putting all my itunes, im, music player in the left screen. So, I do have a more active screen, and that's okay, because I maintain quick access to all of the other stuff I need on the left screen.
2. Approach: Productive Split
The productive Split consists of having e.g. an IDE such as Eclipse running in one Display while Watching output, reading documentation or doing anything else task-related on another screen. Doing this minimized the number of window arrangements massively, speeding up development. Of course, this can also be applied to any other IDE concept, not Eclipse-bound ( using TextMate here too, just nice ).
That's it for now, just a few hints.