But sometimes both Matthew and Luke go for the same half of the dualism. Boom. There goes your argument.
Fair enough. Either is possible, Mark adding to Matthew or Luke or combining the two, or Luke and Matthew trimming down Mark.
What do you think about this:
FAVORITE EXPRESSIONS OF MATTHEW AND LUKE
The unusual Greek phrase 'opsías dè genoménes ("and evening having come"), common to Matthew and Mark in the above example, can be considered a "favorite expression" of Matthew, for the exact wording appears six times in his gospel. In Mark, however, the parallel verses use this wording only the first time; each subsequent passage shows some variation:
Mt 8:16 - 'opsías dè genoménes // Mk 1:32 - 'opsías dè genoménes
Mt 14:15 - 'opsías dè genoménes // Mk 6:35 - kaì 'éde `óras pollês genoménes
Mt 14:23 - 'opsías dè genoménes // Mk 6:47 - kaì 'opsías genoménes
Mt 20:8 - 'opsías dè genoménes // Mk (no parallel)
Mt 26:20 - 'opsías dè genoménes // Mk 14:17 - kaì 'opsías genoménes
Mt 27:57 - 'opsías dè genoménes // Mk 15:42 - kaì 'éde 'opsías genoménes
On the Markan hypothesis, the critic is asked to imagine that Matthew copied this expression exactly as Mark had it the first time he encountered it, but thereafter whenever he followed Mark in the use of this rare expression he consistently deviated from Mark and rigidly restricted himself to this particular grammatical form, even introducing it in 14:15 where Mark had `óras instead of 'opsías, and in 20:8 where there was no Markan parallel. If Mark were secondary to Matthew, however, the critic would only be required to imagine that Mark tended to modify this Matthean expression quite freely whenever he found it in his text of Matthew.
http://www.maplenet.net/~trowbridge/farmer.htm