I was going to post this yesterday, but I couldn't find sufficient evidence to back up an answer and didn't have time to continue researching. But I think it's probably right. Coming back to the question I see bigedsenior has made a similar suggestion (though I can't see any evidence for it in the Google images he's posted).

In principle, given that "rasgadura" basically means a tear in cloth, it seems reasonable to think that it might be a narrow aperture in the "fabric" of the wall. And those who've looked at Romanesque churches will have seen such features. The difficulty is finding evidence to support it.

However, there are some references that point that way. This is on a church restoration project where they have introduced a long, narrow window to give an "haz de luz":

"Surgió así la idea de que un haz de luz, partiendo en apariencia de la linterna, descendiera hasta el tabernáculo, rasgando para ello el muro de mediodía [...] Y como el realzamiento formal de la significación del Santísimo quedaba confiado a esa rasgadura [...] Fig. 24.—La vidriera de mármol que rasga el muro de mediodía"
http://informesdelaconstruccion.revistas.csic.es/index.php/i...

Another restoration project:

"Se había proyectado una rasgadura vertical, a modo de ventana y cerrada por medio de carpintería y vidrio, a modo de pieza en la transición entre ambos alzados."
http://www.academiadelpartal.org/files/n6_227.pdf

More tellingly, I think, there are these two references in a glossary on medieval castles, referring to embrasures or arrow-slits:

"ARQUERA, SAETERA: Rasgadura que se ensancha hacia el interior (también a veces hacia el exterior), y está practicada en los muros, torres y puertas, así como en las almenas, para poder tirar sobre el enemigo con armas arrojadizas. [...]
CAÑONERA, TRONERA: Rasgadura adaptada al empleo de los “truenos” o primitivas armas de fuego. Tenía en su parte baja un agujero circular u ovalado del tamaño adecuado al calibre del arma utilizada. Más tarde tomó forma rectángulo o con arco rebajado con derrama inferior, para aumentar la eficacia del tiro."
http://www.guillenderohan.com/ACCESITGRII/MEMORIACASTILLO/ca...

I think this is enough to conclude that "rasgadura" is a long, narrow, vertical opening in a wall. So "doble rasgadura" very probably refers to double slit windows:

"The apse is the largest and the most richly decorated in sculptures of the Romanesque apses preserved in Andorra; it has two double slit windows and the quarter sphere roof is made of pumice stone."
http://www.andorralavella.ad/en/node/193/sant-esteve

"There are no openings on the first floors, single slit openings on the second floors, double slit windows and stone panels on the third floors."
http://focus.nps.gov/pdfhost/docs/NRHP/Text/08000607.pdf

"The main door was in the centre of the west wall, and slit windows pierced the centre of each of the longer sides of the building, the apse and the east wall of the bay south of it" [this is on Romanesque]
https://books.google.es/books?id=BgQ6AAAAIAAJ&pg=PA93&lpg=PA...


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Note added at 1 day7 hrs (2015-09-03 04:22:19 GMT)
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"typical Romanesque slit windows"
http://www.learn.columbia.edu/ma/htm/kd/ma_kd_discuss_sanl.h...

Many examples of "slit" can be found in English sources on Romanesque architecture. I think it's the word you want.

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Note added at 2 days17 hrs (2015-09-04 14:54:04 GMT) Post-grading
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Thanks! By the time I got round to answering I think everyone had move on :)