How to open a mdi child form from another with vb in the main mdi parent?
In the first MDI child you can create an instance of the secon MDI child, set the MdiParent of the second instance equal to the MdiParent of the first and show the child.
So in the first Mdi child the following code will show a second MDI child
Dim mdiChildForm As New MyMdiChild
mdiChildForm.MdiParent = Me.MdiParent
mdiChildForm.Show()