Obviously there are different ways of doing it (
described here as "kitchen table poly" vs. "parallel poly"—not sure if this was the first place to coin it or what) but one thing that I discovered along my own journey is that the two don't really mix that well in practice. I prefer the "kitchen table poly" in that I like to be friendly with metamours, and I find it least complicated when my own multiple partners are willing to hang out together and be friendly at the same party or whatever.
I don't think the other way of doing it is WRONG, just DIFFERENT, and it was not a good fit for me when my polycule contained someone who was very far on the opposite of the spectrum. Neither I nor my former metamour felt comfortable, with each wishing for and pushing for the relationship shape that we were happiest with.
The types of problems that would come up were stuff like my former metamour would feel left out because we would both be invited to all of our shared partners' social events, but my metamour would opt to not go because she wasn't comfortable sharing space with her partners' other partners (not just me, but any number of his partners), while I was comfortable in those circumstances, so I "got to" go to everything. She would request that I (or other partners) sometimes not be invited so that she could go, which I and our shared partner felt like was unfairly limiting.
In the end, I was ready to leave to eventually move on to a situation with less inherent tension, and the hinge decided that what he wanted was closer to what I wanted, so he followed me out the door. I'm not saying that people can't make the mismatch work, but compiled with our newness to poly and the other challenges that entailed, it was too much for us personally.
Since then, we have made a point of dating people whose desires on the topic are more in line with our own, and we have had some really great parties and events where an entire constellation of partners, FWBs, and platonic friends have commingled, sharing cuddles and love and affection in whatever way we and they desire. Once in a while, it can be awkward or provoke "wibbles," but working through the awkwardness and fleeting bad feelings has been TOTALLY worth it to get to experience the joy of having everyone all in one place having fun. While I wouldn't call either of my current metamours my BEST friends, we definitely get along and have lots of fun together.
ETA: I think I actually do have more regular conversations with my female metamour than I do with my actual best friend—friendly conversations about other people we are interested in dating besides our shared partner, about pets, about travel or work things. We've even taken a long-distance trip together, just the two of us. And this after a bumpy start to our connection due to some jealousy issues on my end. So if friendship is a goal, as long as both people share that goal, it is totally workable.