Dim subFrm As Form1 = Nothing

    Public Sub New()
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.

        subFrm = New Form1(TextBox1) With {.Text = "Подчинённая форма"}
        subFrm.Show()
    End Sub

    Public Sub New(ByRef tbForm1 As TextBox)
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.
        
        tbForm1.DataBindings.Clear()
        TextBox1.DataBindings.Add("Text", tbForm1, "Text")
        tbForm1.DataBindings.Add("Text", TextBox1, "Text")
    End Sub

    Private Sub Form1_LocationChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.LocationChanged
        If subFrm IsNot Nothing Then
            subFrm.Location = New Point(Me.Location.X + Me.Width, Me.Location.Y)
        End If
    End Sub

Dim subFrm As Form1 = Nothing

    Public Sub New()
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.

        subFrm = New Form1(TextBox1) With {.Text = "Подчинённая форма"}
        subFrm.Show()
    End Sub

    Public Sub New(ByRef tbForm1 As TextBox)
        ' This call is required by the designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.

        'tbForm1.DataBindings.Clear()
        TextBox1.DataBindings.Add("Text", tbForm1, "Text")
        'tbForm1.DataBindings.Add("Text", TextBox1, "Text")
    End Sub

    Private Sub Form1_LocationChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.LocationChanged
        If subFrm IsNot Nothing Then
            subFrm.Location = New Point(Me.Location.X + Me.Width, Me.Location.Y)
        End If
    End Sub

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            Form1 myForm = new Form1(ref textBox1);
            myForm.Show();
        }

        public Form1(ref TextBox myText)
        {
            InitializeComponent();

            textBox1.DataBindings.Add("Text", myText, "Text", true, DataSourceUpdateMode.OnPropertyChanged);
        }

    }