As i understand it, when departing Belarus, you'll be asked to account for your time in Russia, Kazakhstan, and Belarus, so you have to have appropriate visas covering all three countries. You'll need to show train tickets, hotel receipts, boarding passes, and migration cards on exit for the whole customs area, at exit from Belarus. You can get a transit visa for Russia for the time it takes you to get to Kazakhstan allowing for 500km/day of travel by the most direct route. Your time in Kazakhstan requires the appropriate visa there, and you can get a Belarus transit visa by showing your itinerary, but you should try to get them all in advance. if you want to spend extra time in Belarus, you need a tourist visa. it's probably easiest to have all of these in your Schengen (Poland) passport, but i don't *think* Belarus will care if your Belarus visa is in your US passport while your other visas are in your Polish passport.