  Classification of the N=1 space-time supersymmetric fermionic Z2XZ2
heterotic-string vacua with symmetric internal shifts, revealed a novel
spinor-vector duality symmetry over the entire space of vacua, where the S_t
<-> V duality interchanges the spinor plus anti-spinor representations with
vector representations. In this paper we demonstrate that the spinor--vector
duality exists also in fermionic Z2 heterotic string models, which preserve N=2
space-time supersymmetry. In this case the interchange is between spinorial and
vectorial representations of the unbroken SO(12) GUT symmetry. We provide a
general algebraic proof for the existence of the S_t <-> V duality map. We
present a novel basis to generate the free fermionic models in which the ten
dimensional gauge degrees of freedom are grouped into four groups of four, each
generating an SO(8) modular block. In the new basis the GUT symmetries are
produced by generators arising from the trivial and non--trivial sectors, and
due to the triality property of the SO(8) representations. Thus, while in the
new basis the appearance of GUT symmetries is more cumbersome, it may be more
instrumental in revealing the duality symmetries that underly the string vacua.
