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This is almost precisely what is to be expected. The copying history of the two Gospels should be different in the first stages, and only should they strongly correlate when they have been combined in a single manuscript, such as a copy of the Four Gospels. Such a copy, like the typical papyrus P75, would then begin a united line of transmission in which the column widths will converge.
Thus for the final leg of their journey, these two Gospels begin to suffer errors with common widths, generated from the same manuscript which is carrying both, and probably two other Gospels as well.