Ideas for long term MFM friendship

Quote by NW30'scouple
We have a great arrangement with a very close friend that’s been ongoing for about three years now. It started when we all shared a hotel room with two double beds and after a night out, we had an mfm which later lead to one on one between my gf and him whilst I watched from the other double bed.
Since then our friend has stayed with us about four times per year and my gf goes to him in our guest room and they have a one on one. We never discuss what we do between the three of us and the only talk that happens is between the two of them in bed and this has generally been about protection issues and whether to involve me in a further mfm, which our friend has said he’d prefer not to do as he would rather continue with the one on one. The following morning nothing is ever said and I have to say that this add to the thrill of the whole situation.
I’m totally happy with this and get huge pleasure from listening outside the door and then the incredible fun when my gf returns to our bed. The ground rules have pretty much been set in an almost unspoken way (if that makes sense) and we are both really happy with our arrangement and my gf and I have a huge amount of fun talking and reliving his visits, as you might imagine.
It is particularly good that our friend lives in London and his visits are frequent enough to keep the momentum going but not too frequent to make the situation boring.
My reason for posting is that we’d love to hear comments from any couples in a similar situation and to hear if they have any ideas or scenarios that they have found have added to the excitement of having such a long term arrangement, as we have been trying to think of ideas that we could try. Thanks!