First allow me to complement you on your English, neither of your previous posts gave the impression that you were a non-native english speaker.
As to your question, Semai grunts can buy anything from the Semai Support list, and Horde anything from the Horde list. Common Support units are on the support list for each Apostle, the Cult, and the Horde, I still think that each side of your army must follow normal buying criteria, therefore even though the Pretorian Behemoth is on the support list for both Semai and the Horde, you must buy his as a support unit for either your Horde forces, or your Semai forces, therefore 2 grunt squads from either list qualify you for a Behemoth, but one from each does not.