So quite by chance I decided to check the date my current Ars Magica rpg campaign began and the answer in May 15th 2020. So happy birthday to the covenant of Avalon, with Wychwood and Wadenholm joining soon after and then Martin's Mere in 2021.
For those who don't know I am running one saga running through the last days of the Angevin Empire in which the Order of Hermes a magical group are attempting to weather the storms (sometimes literally) of history. The game is set in England, the Stonehenge Tribunal, and each group plays on their own day with occasional sessions like the septannual Tribunal gatherings when they can all meet up.
So I am sure lots of people have run D&D campaigns with multiple parties in the same world? What is different here is that Ars Magica is a game about research magicians, writing books, reading books, inventing spells and experimenting to change the Laws of Magic. The result is they want to communicate, boast, and beg borrow and steal resources.
Fortunately the Order has redcaps, who act as messengers, and as we are playing online we can send emails or Zoom or Discord. I as GM act as mailbox and ensure that letters reach their destination unless there is a good reason.
Now in my first campaign the players complained the Redcaps never brought them mail. Now they get plenty, and they really enjoy conducting correspondence with other covenants.
So apart from that what have I learned? Well the magi are loyal to the people they live with, and tend to put House interests second to their covenant. No surprise there! One thing I am keen to do is to recruit players who don't need to attend sessions but represent the Primus of a House, and then have them play out their own political agendas by corresponding with members of their Houses in Stonehenge. Would be an ideal role for someone who has read a lot of the books but never gets to play.
Secondly Hermetic politics and Tribunal meetings are much more interesting once you close on 30 players. If 20 players turn up and their future games will be affected by others votes they will pay a lot more attention than it is just the referee playing everyone there.
Also we can run some large themes. While most of what happens is at the local level, many events have required cross-tribunal cooperation.
And then -- magi change covenant more than I would have expected. They live to visit each other, they love to guest star in each others games and they enjoy exploring the wider setting. Cassius of Avalon and Bloody Bill the Redcap have spent long periods living at Wychwood - some doubt Cassius will return. Gelique was kicked out of Wychwood and adopted by Avalon after she led the visiting magi in to a fairie forest where they met some wolves Wychwood had a pact with... Well let's just say it was bad.
The fevered pace of research and writing and trading is like nothing I saw in my single covenant sagas. And the players have their own group chats for planning and plotting, and I am kept busy adjudicating rules questions. Long periods of sessions just pass in planning and being in character: this is a long term game, and all the magi are endlessly improving. More covenants accelerates the pace.
Anyway just a thought after a year of the game: I will throw it open now to the players to comment and then will try to incorporate their comments in to my final essay. Happy Birthday saga!