I take it these are both AB flights? Just as a matter of reference: last year I flew AB MIA-TXL and our flight arrived almost an hour behind schedule because of a delayed departure. When I got off the bus (remote stand parking) and entered the building to go through immigration, there was an AB representative there asking for the passengers transferring to St. Petersburg. I don't know if they took them directly to the gate airside, or just ushered them through immigration and security. Since you are coming from a non-Schengen country and departing to another non-Schengen country, I'm not sure if you actually need to go through immigration. For example, you wouldn't have to do that at AMS or even FRA since those flights are clustered in the same terminal areas.