The format is called "Permenant Alliance" or "Later Formed Team". Basically game starts as an 8X8 free for all with the option to team up later. At any point of the game a player may propose a permanent alliance to another player. Mostly to one of his neighbors. If they agree, they will be a team just like a regular team. So they will start to see each others troops and they will be able to teleport to each others cities. If they win, they will share the glory and ladder points. You can only make alliance with one player.
This idea came to me when playing a simultaneous game. Me and my neighbor, we keep bashing our troops for one little back city at the hill. It is a hero slaughter-fest. Even a long term alliance wouldn't cut it be cause the city is strategically important. If you don't have them you will be expose to attack from the back and loose your production cities eventually. So if I can make a team with him, that would be better for both of us.
Also in team games, having a partner at the far corner is a huge disadvantage. Having a partner right next to you or better yet just one opponent apart is a huge advantage. It is also a disaster for the opponent in between two partners I may add.
So I would like to have an opportunity to choose my partner after the game starts.