This is the kind of pretentious term critics use. "Página" just means a piece or composition; a score, if you like. See here for example:

"La obra mozartiana es feliz y sus motivos principales se repiten novedosamente lo que tiene que ver con la longitud de la página."
http://www.mundoclasico.com/ed/documentos/doc-ver.aspx?id=01...

It doesn't mean literally how long the page is, but how long the piece is.

"Concertante" is a technical musical term, so I think it should probably be preserved:

"A concertante part in a piece of music is a part that calls for some element of solo performance, as in a classical concerto. The word is found in the phrase Sinfonia concertante, which is used to indicate an orchestral composition with two or more solo instruments, a title used from the late 18th century onwards."
http://www.naxos.com/education/glossary.asp# (click on "concertante")

See also http://en.wikipedia.org/wiki/Sinfonia_concertante

"Concertante piece" is quite a standard expression. See examples here:

https://www.google.es/search?num=100&site=webhp&source=hp&q=...

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Note added at 13 mins (2013-05-16 09:42:01 GMT)
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One point to bear in mind here is that the use of the term "concertante" implicitly relates Chopin's first piano concerto, which is after all a romantic concerto, post-Beethoven, to the earlier classical tradition leading up to Mozart, when the solo concerto was in its infancy and just emerging from the sinfonia concertante. This is presumably deliberate, and that is a reason for keeping the term "concertante".