The Druchii and the Dawi’Zharr share much in common - economies built on slavery and dominance of the sea, and a penchant for black sorcery and cruelty. These two great powers were undoubtedly rivals in their affairs, but like the Golden Age between Elf and Dwarf, there also exists the possibility of a meeting of the minds - an Obsidian Age when trade and communication flourished between the two dark empires.
The first knowledge of the other empire probably came through the tribes of the Chaos Wastes, or else through rumors spread in Cathayan slave caravans. Actual contact is unlikely before 1,000 I.C., when the great canal was completed and gave the Dawi’Zharr access to the Sea of Chaos - before this date, the Druchii ships would have had to venture past the Fortress of the Dawn ‘round Lustria, or otherwise make their way to the Scalded Delta and venture up the River Ruin.
The possibilities for peaceful contact are few, but both Chaos Dwarfs and Dark Elves are renowned for their greed, and the Witch King knows something of Dwarfs of old. Once the existence of their empire was established, and its extant guessed, it would not be too surprising if a Dark Elf ambassador came bearing a message in ancient, if passable, Khazalid. Here at least is a basis for trade: both empires are greedy for slaves, and the exotic slaves both empires had to offer would have enticed traders from the other. Beyond this, the skill and craft of the Chaos Dwarf equals or exceeds that of the Druchii, and much as the High Elves valued the Dwarfs’ crafts, the Dark Elves would likely appreciate the high-quality arms and armor that their precious metals, gems, and dwarf-leather would buy them. Beyond this, the Dwarf treasures and slaves captured by the Druchii would be especially prized by the Chaos Dwarfs.
Once trade was regularly established, the prolonged contact might cause other relations to develop - the exchange of ideas, and alliances of convenience. Both Chaos Dwarfs and Dark Elves share a knowledge in dark sorcery, and although forbidden it would not be unusual for some learning to spread between the two on the sly. This at least might explain the Chaos Dwarfs’ use of Elf-derived language in their magicks. Together, the two navies could have launched joint ventures against the Chaos Plaguefleets that preyed on Druchii and Dawi’Zharr shipping alike, or joined together to prey on the coasts of Brettonia, where the smaller Chaos Dwarf steamships would have an advantage in navigating up the small rivers of the Coast, and the Dark Elves would have the advantage in speed at sea. Perhaps they even planned a joint siege of Ulthuan, or the Fortress of the Dawn.
Whatever the agreements between Druchii and Dawi’Zharr, this dark Obsidian Age is unlikely to have lasted much past 2,000 I.C., when the Sea Elves reinvested themselves in Marienburg. The presence of regular Sea Elf/High Elf fleets in the Old World would inversely affect any Dark Elf/Chaos Dwarf sea traffic in the area, or perhaps was symptomatic of less the same. Whatever the case, the larger presence of the Sea/High Elves suggests that the Obsidian Age was at an end.
The end of regular trade could be for many reasons. Both sides are naturally treacherous and prone to anger, and any sufficient perceived insult would be sufficient to end normal relations, no matter how profitable. Both races are theocratic in nature, and could see the others as apostates and heretics, particularly if the cults of Slaanesh attempted to extend themselves into Uzkulak. Whatever the cause of the falling out, the breach is likely irrevocable - though that would not stop the occasional illicit trafficking between the two.