Herbert,
"I doubt, w[h]ether the music was composed for singing. The rhythmic contents of the voices are to[o] different to permit a common text."
There is an enormous amount of music, especially before 1800, were there is not a "common text," as I take your meaning.
Attached are two bicinia, one, by Orlando di Lasso, 1577, using an actual text, and another, by Francesco Guami, 1588, shown as sung using solemnization.
I think this answers your doubt.
I enjoyed the opportunity to research this -- it's been decades since I've delved into this in depth.
Regards
Walt
PS These two examples are from a PhD thesis, "Two-part Didactic Music in Printed Italian Collections of the Renaissance and Baroque (1521-1744)," by Andrea Bornstein, found at
http://etheses.bham.ac.uk/677/1/Bornstein01PhD.pdf
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