It isn't necessary that metamours be friends, but most people seem to think that it helps if they at least meet and know each other, or have a respectful acknowledgement of each other. Yet, some people have vees that function quite well when both partners have never met in person and only know each other through emails or secondhand messages.
Again, just like the privacy issue, it depends on the people involved and what they are comfortable with. One of my early poly experiences was with a guy who had an agreement with his wife that they both come along on a first date for either one of them. Just to meet. Another guy I was seeing long distance came to my city and spent a few days with me, though the only contact I'd had with his wife was two emails.