How do I delete one of two Ubuntus on my PC

You could use OS-Uninstaller.

You can install it by running these in your terminal window.

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y os-uninstaller
os-uninstaller

Follow this link.

1. Boot the Ubuntu installation that you want to keep.

2. Use your favourite partition manager (e. g. Gnome Disks, KDE Partition Manager, GParted) to delete or format the partition(s) associated with the Ubuntu installation that you want to remove. This will destroy all the data on these partitions.

3. Run sudo update-grub to update the boot entries in Grub.