No- the W4 doesn't need to reflect anything, to be frank.
W4s aren't tax returns. Many people claim 0 and have kids, a house, college, etc. The number of allowances on that form is for withholding purposes and little else. Allowances aren't compared with kids on your return.
Since they are both your children (by marriage or biology), you can choose for one person to claim both. The W4 isn't standing in you way!