The Magento Marketplace is designed so that each package file is exactly that: one package.
That means we can't just make up our own packaging scheme to mish-mash together hybridized franken-extensions into the same zipfile, and expect them to work with the Marketplace's automated testing toolchain. It wouldn't have any idea how to test any of that! So we have to stick to the packaging specification.
But not to worry: if we want to sell two separate extensions as a single package, there are ways to do this for Magento 2!
Option 1: Primary package with shared package dependency
Upload the "less significant" of the two (say, the one in your folder "Mkt") as a shared package (https://docs.magento.com/marketplace/user_guide/sellers/shared-packages.html), which is the Magento equivalent of a "library". This shared package will not be visible on the Marketplace store for purchase separately.
Upload the other (the one in your folder "Abandon") as a regular extension, and have it require the shared package. Then when people buy the "Abandon" extension, composer will automatically download and install the "Mkt" dependency for them.
Option 2: Metapackage that includes both other packages
First, upload both extensions as shared packages.
Then, create a "metapackage" (https://marketplacesupport.magento.com/hc/en-us/articles/360015982231-Metapackages) that is little more than a composer.json that requires both of those shared packages as a dependency. The shared packages will automatically be downloaded and installed by Composer when someone buys and installs the metapackage.
When you sell a package through Magento Marketplace, it automatically grants your customer access to download any of your shared packages that are marked as dependencies, too.