migrating photos to desktop from cloud without replacing old/alternate library currently on desktop

You can have several Photos Libraries. You just have to name them differently to keep them apart.

Using multiple Photos Libraries is explained on this Help page, see: Create additional photo libraries in Photos on Mac - Apple Support

To create a new library for iCloud,

• launch Photos while holding down the options key ⌥. Keep holding down this key until the Library Chooser dialog appears.
• Select "new" to create a different library. Give it a different name from your main Desktop Library.

To make the new library your iCloud Library see this page: Use iCloud Photos to store photos in iCloud - Apple Support

• While browsing the new library: First open the Photos > Preferences > General and click "Use as System Photo Library", because only the System Photos Library can sync with iCloud.
• If you’re not already signed in to iCloud, choose Apple menu  > System Preferences, then click iCloud and sign in with your Apple ID and password. 

1. In the Photos app  on your Mac, choose Photos > Preferences, then click iCloud.
2. Select the iCloud Photos checkbox.

To switch between your Libraries, double click the library in the Finder that you want to open in Photos.