Set focus to another Control

add this to your form

protected override void OnKeyDown(KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Enter)
        {
            Control NextControl = this.GetNextControl(this.ActiveControl, true);
                while (!NextControl.TabStop || !NextControl.Enabled || !NextControl.Visible)
                {
                    NextControl=this.GetNextControl(NextControl, true);
                }
                NextControl.Focus();
        }
        else
        {
            base.OnKeyDown(e);
        }
    }

First, you will have to set the KeyPreview property of the Form set to true. Then you will have to override the form's OnKeyDown method and make a case like:

if(e.KeyCode == Keys.Enter)
{
      Control ctlNext = this.GetNextControl(this.ActiveControl, true);
      ctlNext.Focus();
}
else
{
      base.OnKeyDown(e);
}

Mind you that this code will work for every control on the form, and move the focus to the next one. If you just want this code to work for the textboxes you could add a check like:

if(this.ActiveControl is TextBox)
{
...
}

Handle the KeyPress or KeyDown event of textbox1 and then call textbox2.Focus().