I've been collecting drawings of scribes, and many of them show a single scribe copying a manuscript on a double-lectern, with the examplar on the top or side section and the in-progress copy on the bottom one.
But... I can see that it would be much more efficient, if you needed numerous copies at the same time, for someone to read out the manuscript and several scribes to be copying it down in parallel and I have seen images of a lecturer and students in which the students are holding copies of books (which, from my research, it appears that they often copied out themselves).
Most of these images are crowded into historiated initials, with limited space, so we can't expect them to show the full extent of ways that manuscripts were copied, but it seems that at least one segment probably copied books one-for-one.
Note, that I have NOT done much reading on how books were copied, so these comments ONLY apply to the bits-and-pieces of imagery that I've collected.
But... I can see that it would be much more efficient, if you needed numerous copies at the same time, for someone to read out the manuscript and several scribes to be copying it down in parallel and I have seen images of a lecturer and students in which the students are holding copies of books (which, from my research, it appears that they often copied out themselves).
Most of these images are crowded into historiated initials, with limited space, so we can't expect them to show the full extent of ways that manuscripts were copied, but it seems that at least one segment probably copied books one-for-one.
Note, that I have NOT done much reading on how books were copied, so these comments ONLY apply to the bits-and-pieces of imagery that I've collected.