The coteries are taught a Sabbat power and told that if they show it to anyone, they will surely be killed. If they ask why, they are told that when they know, they can come back to learn more.
Each player gets one secondary level 1 power that a player in the other coterie chose. This is what interlinks which player has taken the other’s place. The circle doesn’t have to be neat. There might also be further bleed between linked players.
A single instance of diablerie can be hidden by the magic that swapped the players.
Sympathetic ability/attack. Stunning attack that shares the pain of the caster.