As mentioned in "BUG: Dual Base Problem"
http://ufo.myexp.de/phpBB2/viewtopic.php?t=233,
you can use the second base as soon as it has an entrance, i.e. no hangar/supplies/quarters etc.
The base also gets a free dropship and a batch of soldiers, and some equipment.
This equipment is linked to the first base:
E.g. you outfit the first 8 soldiers of the second base with armor etc.,
then you send the team from the first base on a mission.
If one of this men die during this mission, his armor is lost.
Now one of the men from the second base is also missing his armor.
