Let's start with the BOE definition in the curriculum for advanced musical studies:

" Concertación (Canto).-Estudio e interpretación del repertorio concertante de ópera, oratorio y zarzuela.
Profundización en las capacidades relacionadas con la comprensión y respuesta a las indicaciones del director, la integración en el conjunto vocal y su relación con el grupo instrumental acompañante.
Concertación (Dirección de Orquesta).-Estudio y práctica interpretativa del repertorio concertante vocal e instrumental (ópera, oratorio, zarzuela, conciertos para dos o más solistas etc.). Desarrollo técnico y de los conocimientos necesarios para la correcta dirección del conjunto de solistas, y la relación de éste con la orquesta o grupo orquestal acompañante.
Concertación de Ópera y Zarzuela.-(Ver Concertación de Canto)."
https://www.boe.es/diario_boe/txt.php?id=BOE-A-1999-14767

So it is part of both the singing and the conducting curricula, and in both cases it refers to the study and practice of "concertante" music, which, as "conjunto vocal" and "conjunto de solistas" indicate, means performance in a group of soloists (i.e. two or more), with accompaniment.

"concertante
1. adj. Mús. Dicho de una pieza: Compuesta de varias voces entre las cuales se distribuye el canto."
http://dle.rae.es/?id=A7oGg4d

It doesn't have to be voices; a "concertante" instrumental piece is one with two or more solo instruments, as in a sinfonia concertante.

"concertante
1: a 17th or 18th century musical composition for orchestra with parts for solo instruments or for several solo instruments without orchestra"
http://www.merriam-webster.com/dictionary/concertante

So then it's about learning to perform with other vocal soloists or to conduct groups of vocal or instrumental soloists, and to coordinate with the accompaniment.

"“Concertación de repertorio teatral y camerístico” está dedicada al estudio y desarrollo de la interpretación musical en grupo del repertorio concertante de ópera, oratorio y canción. [...]
· Interpretación musical en grupo del repertorio concertante de ópera, oratorio y canción.
· Ajuste de los recursos técnicos y musicales individuales a las necesidades estilísticas e interpretativas de la obra de conjunto.
· Integración en el grupo. [...]
http://www.csmmurcia.com/2014-2015 Gu�as docentes/Concertaci�n de...

Well, I think we would call this ensemble singing or ensemble conducting. It's true that the word "ensemble" can refer to a group of musicians of any size up to an orchestra, band or chorus, but it will be understood that it doesn't refer to these, which are referred to by as such in conservatoire studies. "Ensemble" will be taken to mean small groups of soloists (two or more) and coordination, which are the two key issues here.

"ensemble 1. A specific group of singers or instrumentalists [...]
2. How well two or more musicians perform together with particular regard to rhythmic synchronicity [...]
3. An operatic or musical-theater excerpt for two or more soloists."
Matthew Hoch, A Dictionary for the Modern Singer, 59
https://books.google.es/books?id=1Y1_AwAAQBAJ&pg=PA59#v=onep...

"Ensemble singing
[...]
It will focus primarily on 'classical ensemble singing, or consort singing as it is sometimes, i.e. one voice to a part (usually no more than about eight parts [...])"
John Potter, The Cambridge Companion to Singing, 158
https://books.google.es/books?id=8T73OnRmIVIC&pg=PA158#v=one...

So ensemble singing is about groups of soloists, as opposed to choir singing.


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Note added at 2 hrs (2016-06-13 16:44:52 GMT)
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"Elizabeth is studying at Birmingham Conservatoire with Catherine Benson and Andrew King, specialising in early music and ensemble singing."
https://joinencore.com/Elizabeth-Adams

The following is interesting because it draws attention to the relative absence of this subject in UK conservatoires:

"Ensemble Singing, Singing Teachers and the Pavarotti syndrome.
British universities (not just Oxbridge) produce a huge number of outstanding ensemble singers, who for decades have sustained successful professional choirs from the Monteverdi Choir (recently described on Radio 3 as the best choir in the world) to The Sixteen and a plethora of creative ensembles from the Hilliard Ensemble to Juice. Conservatories don’t really do ensemble singing (being still geared to the mass production of conventional soloists)"
http://www.john-potter.co.uk/blog/2010/12/30/ensemble-singin...