Can a unit join another unit if they are the same type of unit?
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.