I think a big thing to consider is how much personal attention the players are wanting/expecting from you. If everyone has a lot of backstory, detailed contact relationships, etc and that is going to play a large role in the campaign, I wouldn't really want to do more than 6 players (personally I prefer 5), but if it's more of a merc/war style setting, or just runs with not a lot of in-between run social stuff you might be all right. Or, as someone mentioned above do all the downtime stuff between sessions, but I would worry that some of the wonderful possibilities for inter-pc relationships developing might get a little lost in this case. Also, this method still would require quite a lot of your time, out of game, to respond to all the individual players. Or, recruit a co-gm (or 2) from amongst the players- maybe have 1 person track initiative and move stuff around on whatever map or grid you use, and have another player on top of looking up rules. And then encourage all the other players to make sure to know the rules they need to play their pc, and to decide on actions/look anything up they need to during other player's actions to keep things rolling. Still, a bit of a daunting task. The 2 groups of 4 sounds potentially really cool (I like the idea of overlapping campaigns a lot), but each group only playing 1/month would be too long between sessions imo. Best of luck!