I know it's not the best option... but you could use the 2 channel output for one track, and the headphone output for the other track. The audio fire has 4 analog outputs,not 2.
When using an analog mixer you'd do the cueing with the mixer, not the sound card.
In the echo console you can select the main outputs to be -10db, and that way you'd have the same level in the 4 outputs. Most CDJ today are -10db RCA connections anyway, not +4db.
I know it's not optimal, but it's waaaaaay lot better than using an external converter to convert spdif. Yeah you get lower volume, but you'd be using the really good audiofire ADDA converters, which is what you're paying for basically.
BTW I had the firebox a few years back and was pleased with it, but I think the echo is much better in terms of ADDA conversion. Plus the audiofire has lots of things you're paying for and you won't need for djing like the preamps.
And you know where the name firebox comes from? Not because it uses firewire... but because after some times it gets VERY hot. You can still handle it but it almost burns. I don't know what would happen in a club environment where the temperature is high and humid... That was the case of my firewire version, I don't know about the usb version.
I'm not saying it's bad interface, only that imo it's not the best option for djing unless you plan to produce and record stuff too.