Can a unit join another unit if they are the same type of unit? If so, what are the conditions for which they can join?
For example, can they join the other unit while the other unit is in close combat?
For example, a unit of spirit hosts that were summoned trying to join another unit of summoned spirit hosts.
No, units can never join together.
Forming Units, pg 5
Once formed into a unit, the models move and fight as a single entity for the rest of the battle.
Moving your units, pg 13
Individual models are not permitted to leave their units and so a unit effectively remains a single entity.
1" Apart, pg 13
Whilst moving a unit is not allowed to appraoch closer than 1" to another unit, friend or foe
The only exception to this are Characters, who may join and leave units (pg 96-101). The rules for characters joining and leaving units are detailed and explicit, there are no such rules for normal units.
