How do I add multiple values to GRUB_CMDLINE_DEFAULT in /etc/default/grub?
Kernel parameters in /etc/default/grub's GRUB_CMDLINE_LINUX_DEFAULT variable are simply separated with just spaces:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash net.ifnames=0"
This sets the three parameters quiet, splash and net.ifnames=0.
Also, as correctly pointed out in the comments, don't forget to update your GRUB configuration with the changed parameters afterwards, by running:
sudo update-grub