You can split your listings on the product catalog level. This is not the same as splitting up marketplace listings, which separates the listing on the marketplace.
When you create new listings thru Listing Mirror, Listing Mirror uses product catalog to determine the variation grouping on the marketplace. So if SKUs A, B, and C are a variation listing in product catalog, then they will be grouped as such when created on a marketplace.
To do this, go to the Product page, locate the Actions section, and select Split Listings (this option will only be visible if you have at least 2 variations for the current listing).
Once the Split Listings page is visible, you have the option to update the existing parent's title, enter a title for the new parent, and select which child listings will be moved to the new parent.
In the above example, I am splitting the 'blue' color, SKU bisselA from the existing product. It will be under the title 'Something Different.'