Well, there’s a “hard way”: telling the updater to ignore the signatures.
Edit /etc/pacman.conf
Search the line:
SigLevel = Required DatabaseOptional
And change to:
SigLevel = Optional TrustAll
Update keyring
sudo pacman -Sy archlinux-keyring manjaro-keyring
Once installed, undo the change made in pacman.conf (restore SigLevel to Required DatabaseOptional)
After that you should have both keyrings installed.