If you go to extended leagues (non-base game setup) like the Faroe Islands, or Estonia, you can have multiple reserve teams in lower leagues. Teams like Flora Tallinn, and most of the top division in the Faroes will have a second team and a third team in the 2nd, 3rd, 4th tiers and they can be used to develop players - as well as the u18/19 squad separately.
The challenge here is that it's unlikely that few of these teams are professional in setup.