I would explain the verbal overlap between passages in Matthew and Luke which are absent from Mark as follows: Matthew and Luke both made use of another source to which Mark either did not have access or chose not to utilize.
This explanation is in line with the 4-Source Hypothesis.
This additional source, now lost to us, is commonly referred to as “Q” for the German
quelle (“source”).
This collection of Jesus traditions was not necessarily a written document but may have simply been a set of familiar teachings which were part of the oral traditions about Jesus circulated in the early Church.
Matthew and Luke sometimes have sections of this source material in the same order with a high degree verbal overlap.
Other times they share the source material but use it in different places in their presentation.
Because of the length and precision in some of these instances of verbal overlap, it seems probable that there may have been a written source to which Matthew and Luke had access, though it is possible that it was common oral tradition which they both incorporated. Just as the verbal parallels between Mark and the other two Synoptics suggests a literary dependence, so too the verbal parallels between Matthew and Luke which are not found in Mark suggests that they both were dependent on an additional shared source.
For example, in the case of the Temptation narrative preserved in Mark 1:12-13 // Matthew 4:1-11 // Luke 4:1-13, Mark gives the temptation only the briefest mention, whereas Matthew and Luke devote quite a bit of space in presenting more fully the exchange between Jesus and Satan. Matthew and Luke are very similar in their presentations (in fact, nearly half of their wording overlaps), although they do place the temptations in a different order. Since Mark includes none of the extra material it is plausible that Luke and Matthew are relying on some other source to which they had access in common.
There are many other instances in which Matthew and Luke display verbal overlap in describing traditions which are not preserved in Mark. When Jesus teaches the people concerning John the Baptist in Matthew 11:2-29 // Luke 7:18-35, both accounts report His teaching with a great deal of overlap. While some have argued that this overlap results from either Matthew depending on Luke or Luke depending on Matthew, the slight variation in the specific wording and details suggests to me that both were relying on a third source independent of one another.
