Almeria won 2-1 at home to Las Palmas after extra time in their semi-final second leg for a 3-2 aggregate victory, while Girona saw off visiting Alcorcon 3-1 for a 4-2 aggregate success.
The first leg of the final is on Wednesday and the return match is on Saturday.
Almeria were relegated in the 2010-11 season and the south coast club finished seventh in the second division last term. Catalonia-based Girona, founded in 1930, have never featured in the top flight.
Alicante-based Elche, the second division champions, and second-placed Villarreal secured the two automatic promotion berths, while Real Zaragoza, Deportivo La Coruna and Real Mallorca were relegated from La Liga.