|Environment||Any vacuum (Material Plane and Ethereal Plane)|
Source: The Rune Drive Gambit, pg(s). 61
The two bodies of a twinsoul remain close across the veil between the Material Plane and the Ethereal Plane. In battle, they can separate and aid each other. The ethereal portion is particularly vulnerable to ethereal attack and force effects. When either part dies, the whole twinsoul will die as well.
Normally, a twinsoul's halves meld with each other daily, but only very briefly. Near a rift between the Material Plane and the Ethereal Plane, they can stay together for an extended time and mate, giving birth to a single spawn. Young twinsouls stay with their parent for several decades before maturing and departing.