That's really the key, but here are few other suggestions you might want to consider.
You need to break up the sight lines at substrate level, because the male will want everything in sight as his domain. If that's the whole tank, that's what he'll claim, but if it's just a corner, even a small one, he'll be content with that as long as he can't readily see the rest of the tank. That's why I suggested Java moss, as it's an effective sight barrier, and you really need a
densely planted tank. The individual 'caves' don't need to be very big:
Nanochromis like to be 'cozy,' and if the pair can fit into it, it's big enough; anything more is just wasted space. Shale is ideal, stacked in such a way that they can dig
under it.
As indicated above, another component of their behavioral repertoire that you must satisfy is their need to dig. Your first substrate was not appropriate for these fishes, and even the second one is coarser that it should be. The digging is an important part of pair bond formation and maintenance, and serves to dissipate energy that might otherwise be channeled into aggression.
Finally, although a small tank like this one is fine for an
established pair, getting them to pair up in such a limited space can be tricky, as you've discovered. Once you've dealt with the tank setup, I'd add both females, and once they've settled in, add the male, and keep a close eye on things. At the very least this will serve to spread the aggression around, and it may increase the likelihood of pair bond formation. With these fishes, that is the biggest challenge. Worst case analysis- you need to give them a bigger tank, perhaps something like a 33L or 40L. Good luck!