I remember playing Championship Manager way back in 1999 and the Brazilian league was split into two different championships and I noticed that the league in Argentina is similar. They both have different names (Apertura and Claustura I think) and I was wondering what the reasoning behind this decision is? Are they considered separate to each other or does your final position in one affect your starting position in the second league?
Note: the answer below is outdated as of 2021, it was valid only until 2014. After that, the Argentinian league again reverted to the European format. See other answers for more recent information.
Actually, the Brazilian league is currently not split. It plays like a standard round-robin league where each team plays each of the others twice.
The Argentinian league is split into two tournaments. The "Torneo Initial" runs from August to December and the "Torneo Final" from February to June. You can take this split literally, as basically it means that if it was a 'regular' European season, the first half would count as part of the torneo initial, while the second half of the matches would be part of the torneo final and thus in each tournament each team would play each other once. With all return fixtures part of the other "Torneo".
In this case, the winners of both leagues play eachother to determine the overall winner.
At the end of each season, the two teams with the worst three-year averages and the worst positioned team in the season table are relegated. Newly promoted teams only average the seasons since their last promotion.
Currently, the Argentine league is not split. Since 2014, there is one tournament again.
Starting August 2014, the "Torneo de Transición" was held, with 20 teams participating (17 from the 2013–14 season and 3 promoted from the 2013–14 Primera B Nacional). No teams were relegated at the end of the championship.
In 2015 the format switched to a tournament with 30 teams. The first five clubs of the Zonas A & B of 2014 Primera B Nacional season promoted to the Primera División. Those 10 teams, with the addition of the 20 clubs currently participating in the top division, qualified to contest the next season.
That same year, the AFA announced the format for the next five seasons of the Primera División:
In the first half of 2016, the league will be contested between thirty teams. Three teams will be relegated to and one team will be promoted from Primera B Nacional. From August 2016 to June 2017, the league will be contested between twenty-eight teams. Four teams will be relegated to and two teams will be promoted from Primera B Nacional. From August 2017 to June 2018, the league will be contested between twenty-six teams. Four teams will be relegated to and two teams will be promoted from Primera B Nacional. From August 2018 to June 2019, the league will be contested between twenty-four teams. Four teams will be relegated to and two teams will be promoted from Primera B Nacional.
As for the reason it was split before in Argentina, it was to have more champions and make the league more profitable (more champions = more merchandising and stuff to sell.)