Or HaChaim on Numbers 32:30/אור החיים על במדבר ל״ב:ל׳

1 ואם לא יעברו חלוצים אתכם, "But if they will not cross with you, armed, etc." The emphasis in this verse is on the words אתכם, "with you," i.e. at the same time as you. If these two tribes were to decide to join you later and help in the conquest they would already have forfeited their claim to special consideration on the East Bank. The reason the Torah wrote ונאחזו בתוככם, "and they will have possessions amongst you," with the added letter ו at the beginning of the word ונאחזו instead of writing יאחזו, "they will receive a possession," is to point out that in such an event they have forfeited their success in this part of the land and they will automatically only inherit their share on the West Bank. They will have failed to perform the מצוה they took upon themselves and therefore will not have attained what they aimed for.