I'm guessing a little here because their stated specs don't 100% match up. What I mean is this: It says input voltage from 11v-30v, 700w per channel, and 30A per channel. 30A at 30v would be 900w, not 700w, so something is getting de-rated along the way. I don't know exactly WHY they are de-rating it, but my educated guess would be that the power components are only good to 30A at the lower end of the input voltage scale. Anyway, lets assume it really can do 30A per channel at the lower input voltages. At 12.4v that comes out to 372w per channel, or 744w total. So, a little more than half of what the top-line nameplate number says. That's actually not bad. Usually you only get about a third of the advertised number. It's still nowhere near 1400w, but that is normal the way they rate these DC/DC chargers.
Now, 372w is enough to charge a 6S, 5000mah battery at almost 3C, so it still isn't going to be an issue unless you are doing a bunch of parallel charging off of one channel. That's the reality of single-battery charging: unless you want to charge high voltage packs at higher than 1C, you don't really need much of a charger to handle one battery at a time. If you are only charging 2S and 3S packs, you could just about use a potato with a couple nails in it.
Now, 372w is enough to charge a 6S, 5000mah battery at almost 3C, so it still isn't going to be an issue unless you are doing a bunch of parallel charging off of one channel. That's the reality of single-battery charging: unless you want to charge high voltage packs at higher than 1C, you don't really need much of a charger to handle one battery at a time. If you are only charging 2S and 3S packs, you could just about use a potato with a couple nails in it.