In the US, this is hardly ever an issue, but the GILM discusses what is to be done if, for pastoral reasons, one of the two readings before the Gospel must be omitted.
79. In Masses to which three readings are assigned, all three are to be used. If, however, for pastoral reasons the Conference of Bishops has given permission for two readings only to be used, [Inaestimabile Donum 1] the choice between the two first readings is to be made in such a way as to safeguard the Church’s intent to instruct the faithful more completely in the mystery of salvation. Thus, unless the contrary is indicated in the text of the Lectionary, the reading to be chosen as the first reading is the one that is more closely in harmony with the Gospel, or, in accord with the intent just mentioned, the one that is more helpful toward a coherent catechesis over an extended period, or that preserves the semicontinuous reading of some biblical book. 
The 106th note gives a concrete example:
For example: in Lent the continuity of the Old Testament readings corresponds to the unfolding of the history of salvation; the Sundays in Ordinary Time provide the semicontinuous reading of one of the Letters of the Apostles. In these cases it is right that the pastor of souls choose one or other of the readings in a systematic way over a series of Sundays, so that he may establish a coherent plan for catechesis. It is not right to read indiscriminately on one day from the Old Testament, on another from the Letter of an Apostle, without any orderly plan for the texts that follow.
It’s a lot of fuss devoted to a situation I would rack my brain for which to come up with a justification. My advice: read the whole given Lectionary.