Multiple Macintosh HD - data drives - Catalina

What you did was to choose the wrong disk on which to start the installation. Backup your files, erase the disk, reinstall macOS from scratch. You can do that via network recovery.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible, back up your files before proceeding.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

Well, the 48 hour install post was not available when I posted my answer. I would not be able to answer that. However, I got what info I have over in the other forums both of us frequent and the Apple article is the bestor closest I have seen. Others have mentioned that deleting the second Data drive while in recovery > Disk Utility is an easy way to deal with it.

True. It was not part of the OP when I first posted my reply, either. The article you linked is a fine article. My original reply suggested using Internet Recovery in order to both erase the disk and install Catalina from a completely separate "device." Disk Utility is the utility of choice for erasing and formatting a drive but provides no attendant way to install macOS. Internet Recovery provides a Utility Menu that includes Disk Utility and the option to install macOS.

I might delete the second Data drive but in this case the first Data drive now has Catalina on it, and the second Data drive has the Data. I don't know what is on the Macintosh HD since there's no information about it. If it has Catalina still on it, then one could try deleting the first Data drive but, definitely, not the second one.

If you do not erase your hard drive first, Catalina will duplicate the Data volume. Apple has published instructions here:

https://support.apple.com/en-us/HT208496

No, it doesn't explain that at all. And, there is not enough information to deal with it. No installation of macOS should take that long.