Well, I think the "halves" thing was a misnomer to begin with, so the potential for multiple fragments is still in play. But insofar as that goes, I think part of Uther's soul was saved by the Light and ultimately sent to Bastion, and part of the soul was taken in by Frostmourne. The part in Frostmourne would've been the source of the echoes that we encounter in ICC and the HoR, and either the Bastion portion or the Frostmourne portion could've (somehow) appeared at his tomb when summoned. Whether or not the portion in Frostmourne was itself fragmented is subject to debate depending on whether or not Frostmourne is itself an extrusion of the Maw beyond the Shadowlands, or if Zovaal was somehow able to tear away soul fragments from Frostmourne's prisoners unbeknownst to its current wielder (like a third party skimming power off the top, we'll say). I think the latter construction is more probable and would explain how we encounter the souls of Halakh and Gavinrad in the Citadel during Legion and why Zovaal has fragments of their souls in his Ossuary.
TL;DR:
- Uther was splintered by the twin forces of the Light and Frostmourne and a portion of the remaining soul in Frostmourne was taken by Zovaal.
- Sylvanas was taken by Frostmourne, had a portion of her soul removed, and the Frostmourne portion removed to become a Banshee.
- The rest of the souls were absorbed into Frostmourne, and fragments of their souls were taken by Zovaal.