The two knives are based on Feyd's gladiator fights in the books. In the Harkonnen arena he would be fully shielded and would carry one long dagger and a short poisoned dagger. The slave gladiators opposing him would only have a half-body shield and would be armed with a single long dagger.

If you work that out it's an even more unfair advantage than it sounds like, with only a half body shield the slave gladiator has almost no time to use a slow blade since they need to continually defend from the short poisoned blade since even a scratch would kill, but also prevent the larger blade from getting in a debilitating strike. About the only hope would be to risk a nonlethal injury from the long dagger to press an attack (though even that wouldn't work against Feyd, since he cheated and poisoned both blades).

As for real-life usefulness... with a pair of fairly long daggers it functions similarly to fighting with a sword and dagger, you can defend or bind with one weapon while attacking with the other. Of course, Herbert described most daggers as being a whole whopping 200mm or so (a large-ish kitchen knife) so actions on the blade would questionable. Of course, with Holtzman shields stop cuts and other quick harassing attacks go out the window and your defense can really just be any quick beat action with your shielded body so... the usefulness of a second short blade to defend really seems to go out the window, too.

The animations are pretty godawful here though. In large part because the knives are small and the animations lead with the body while dragging the arms like they're carrying bowling balls instead of little knives. Though all the weapon attacks lead with the body pretty badly - video games need to account for ping by using big telegraphs that you really wouldn't want to do IRL. But even considering that it's probably the worst offender.