When using CLI, you can run kopia command in parallel as many times as you’d like, each can target the same or different repository (which is stored in a config file, specified using --config-file).
So you can run:
$ kopia --config-file first.config snapshot create /some/path
$ kopia --config-file second.config snapshot create /some/path
When using UI, snapshots are taken automatically on schedule up to Maximum Parallel Snapshots that you can specify in policies. KopiaUI supports N concurrent repository connections.
As for the password, you can always pass it using KOPIA_PASSWORD but there’s usually no reason to do so, when you use kopia repository connect the password will be stored in OS-specific keychain (Windows/macOS) where it’s encrypted.
On Linux this is disabled by default since it only works on Gnome and breaks CLI scenarios so you need to pass --use-keyring flag.