You even cannot go to Russia and not see those cities, because the amount of historical and cultural sites in Moscow is fantastic and St. Petersburg with its beauty and special character is one of the most romantic cities in Europe. You can spend easily a month in each city and still find not discovered attractions.
At the second hand you may not limit yourself with Moscow and St. Petersburg only, as Russia is the largest Country in the World, that covers 1/6 of the Land on Earth planet. There are numerous Towns and Villages, Forrest and Mountains, Lakes and Rivers to discover. The most unique are the towns of the Golden and Silver Rings of Russia – Suzdal, Vladimir, Pskov and Veliky Novgorod. The distant destinations are also worth to see, such as Murmansk, Karelia, Kazan, Sochi, Lake Baikal, Altai region and Kamchatka.