(I'm haven't forgotten about that question about the link between Aleph and B. In the meantime . . .)
Can a manuscript of Matthew attest to a reading in Luke? Can a manuscript of Luke attest to a reading in Mark?
Sometimes. If a MS of Matthew has the Eusebian Canon-Tables, they might be in a form which includes the section-numbers of, say, Luke 22:43-44, and 23:34. The text won't be there, but their echo will still reverberate in the Canon-tables. A closer study of the differences in forms of the Canon-tables might show that one can discern the presence of non-extant readings even with only the section-and-canon-numbers in the margins, without even having the Canons themselves. (Assuming that the Canons reflect the actual contents of the manuscript, and weren't taken from a secondary exemplar.)
And consider the ending of Mark as represented in MS 2882, a manuscript that is housed at the CSNTM in Texas. It contains most of Luke, and no text from Matthew or Mark. But in its margins in Luke 24, there are references to the Heothina-series. See the margin-reference to the fourth reading at
When one has a margin-reference to the fourth Heothina, it implies that the MS, when complete, contained the other readings in the series as well, including Mark 16:9-20 (the third reading in the series). So: could it be said that there are more MSS that support the inclusion of Mark 16:9-20 than there are MSS of Mark?
Yours in Christ,
James Snapp, Jr.