Grand Unification with Higher Rank Product Groups
  Various ideas support the notion that the GUT gauge group might be a semi-simple direct-product group such as $SU(5) \times SU(5)$. The doublet-triplet splitting problem can be solved with a direct product group. String theory suggests that the GUT scale is a modulus. Requiring this rules out a single SU(5) gauge group. A model with $SU(5) \times SU(5)$ gauge symmetry and the GUT scale as a modulus has been shown to exist. It is shown that extending these ideas to $SO(10) \times SO(10)$ cannot be done with the above requirement without unwanted massless modes at lower energy scales that spoil the unification of couplings. Therefore these two conditions highly constrain the class of possible GUT models. 
Erik Kramer