Good news: our fixtures come with L-brackets that fit into a channel on the top of the fixture and slide up and down the length of the light, so you will be able to suspend a 24" light above your tank...and this will be a better option that the 18" fixture as it runs 30 LEDs rather than the 15 in the 18". I don't think an 18" will adequately and uniformly cover 24"-wide tank unless you planned on keeping very low-light, undemanding species on the ends.
Is there any reason you need to suspend the light fixtures so far from the surface? You'll be losing a significant portion of your light simply due to distance, and would do much better to hang the lights a few inches above the surface of the water. If you do go with such an elevated suspension, you're likely to achieve only medium lighting even if running two XB fixtures. If you keep the fixtures closer to the water, you can easily achieve high light parameters.
As far as spectrum, our most popular combination is the 6300K with the 10,000K. I'd get the regular and not XB 10,000K if you went with that option; if you prefer sticking with only 6300 fixtures then two XB's should work fine. To my eyes, the 6300K renders colors in the red/pink/orange group far better than the other spectrums, but if you prefer cooler lighting you may prefer the 10,000K.
Hope this helps, and let me know if you have any further questions!