The amount of fatigue each slave receives depends on their current path level relative to the master's current path level without the communion path boost.
If Master path / 2 is higher than Slave path ⇒
The slave receives fatigue cost x 4
(quadruple fatigue cost)
If Master path is higher than Slave path ⇒
The slave receives fatigue cost x 2
(double fatigue cost)
If Master path is equal to Slave path ⇒
The slave receives fatigue cost
(same fatigue cost)
If Master path is lower than Slave path ⇒
The slave receives fatigue cost / 2
(half fatigue cost)