Continuous archiving: is it possible to use multiple machines?
First, I always create a wrapper script when creating commands like that. That makes it easier to debug the command without having to signal postgres to reload anything. This also makes it easier to include logging from your archiver script.

What you need to figure out first is how you want your data replicated. The way you're doing it now seems ok, at least if you check if the destination file doesn't exist before copying it. That way it can be re-run as many times as you like and it will try to copy it to the external location.