Bereishis 48:6 refers to what the status of any future children after Ephraim and Menashe that Yosef would have had would be:
וּמוֹלַדְתְּךָ אֲשֶׁר-הוֹלַדְתָּ אַחֲרֵיהֶם, לְךָ יִהְיוּ; עַל שֵׁם אֲחֵיהֶם יִקָּרְאוּ, בְּנַחֲלָתָם
And those born to you after them will be yours. They will be called by the names of their brothers in their inheritance.
Ephraim and Menashe later got separate sections of the land of Israel and inherited as separate tribes. So how practically would the future inheritance of these brothers have worked? Would they have been assigned to be split between either Ephraim or Menashe, or would they somehow have taken from both (Ephraim and Menashe had bordering portions in Israel)? If there would have only been one additional son, seemingly he would have had to divide in somehow.