It doesn't matter for how many handlers your TextBoxes_Enter
method will be assigned - it will behave exactly the same way for each one of them. If it doesn't work, it means you're talking about 2 different methods. I almost missed the fact, that your:
private void TextBoxes_Enter(object sender, EventArgs e)
should be
private void TextBoxes_Enter(object sender, KeyEventArgs e)
BTW, you can assign them just like this:
control.KeyDown += TextBoxes_Enter;