To make it work, we bought dedicated iPads and put them on stands next to our computer monitors on our desks. We start a FaceTime call at the beginning of each day and literally leave it running all day. We mute when not actively talking, so we can listen to music (and not listen to the other's keyboard noises).
It is amazing. We always can see if the other one is present and/or busy on a phone call. If I want to talk, I just unmute myself and start talking. It is surprisingly similar to being in the same room as someone, without having to agree on the playlist.
Obviously, this particular setup doesn't scale to larger teams, and it does rely heavily on the very high quality and reliability of the audio and video of FaceTime. But I'd recommend it highly for those in the same situation. Made the cross-country move possible without destroying the business.
The dedicated device is key, so that it is always visible and doesn't take CPU or screen real estate on your actual computer. And I can't emphasize enough the importance of the high quality audio in FaceTime. We tried Squiggle back in the day and the audio/video were just not good enough (and the freeze frames were unflattering and distracting in a way that silent video is not, strangely). It would be nice to unmute both sides at once, but FaceTime is obviously not going to add that feature.
We thought about trying to make a product out of our insights/experiences but concluded that it would be a bit like re-implementing MS Word to fix the find dialog (if anyone remembers that issue back in the day).