Complex sentence: The new constitution of the Russian Federation accepted on 25 December 1993 separated those two entities; since then, head of that office took the official title "Chairmen of the Government", or unofficially just "prime minister".
Split into simpler sentences: The new constitution of the Russian Federation, which was accepted on 25 December 1993, separated those two entities. Since then, the head of that office took the official title "Chairmen of the Government", or was unofficially referred just as "prime minister".