No, there’s no unmerge option.

To merge listings, hover over a listing, select “Merge,” and then choose the corresponding listing or listings in your other shop or shops. This helps keep products synced across channels. For a detailed walkthrough, see the Help Center video on merging listings.

They become linked in Vela for side-by-side management, but edits still only apply per shop.

Not yet. Merge lays the groundwork for inventory management, which will come in future updates.