This is a method to make simultaneous downloads at Vimm
Step 1: Download the Free Download Manager Extension (make sure you have FDM installed) - https://chromewebstore.google.com/detail/free-download-manager/ahmpjcflkgiildlgicmcieglgoilbfdp
Step 2: Once the extension is enabled it will automatically make downloads in the FDM application in which you can make simultaneous downloads.
NOTE: To disable it, click on the extension icon and click manage extensions. From there you can disable the FDM extension, you can enable it whenever you are going to make downloads in Vimm.