On 14 May, the Legislative Assembly of El Salvador passed the required measure to organize the governance of the chamber for the coming term, following the elections this past March. It is a rather unusual arrangement: The second and third largest parties will each hold the assembly presidency for half of the three-year term.
Lorena Peña of the FMLN will be assembly president for 18 months, and then on 8 November 2016, Guillermo Gallegos of GANA will assume the assembly presidency for the remainder of the term.
The opposition ARENA won 32 seats (39% of 84), against 31 (37.8%) for the FMLN and 11 (13.4%) for the GANA. The latter party has its origins in a split in the right-wing ARENA that occurred following the election of the first president from the left-wing FMLN in 2009. It has been an ally of the FMLN ever since. ARENA partner PCN won 4 seats on its own in the election, and another three seats were won by ARENA-PCN alliances in various constituencies. Therefore, the actual balance of the assembly is 42 FMLN+GANA (51.2%) to 39 ARENA+PCN (47.6%); one seat was won by the Christian Democrats (PDC).
The directing board (junta directiva) will have 4 members each from ARENA and the FMLN and 3 each from GANA and the PCN. This body, then, will have equality between the two blocs, thereby slightly over-representing the right-wing bloc. I do not know the actual powers of the presidency vs. the directing board.
I certainly do not claim exhaustive knowledge of how power is divvied up in legislative organization around the world, but the alternation of the presidency for equal time periods between two parties that are quite unequal in strength (and second and third in seat totals) must be a rare occurrence.