This depends on what you mean by: No user intervention
It also depends on which OnUpdate your talking about.
If what your saying is that you have code in your second Form's OnUpdate that you want to be called by your first form then you could do one of two things:
1. Remove the Private declaration from your OnUpdate in the second form and call it directly from your first form's code:
'Form1's module
Sub mySub()
doCmd.OpenForm "Form2"
Forms.Form2.Form_OnUpdate
End Sub
'Form2's module
Sub Form_OnUpdate
'Your Code here
End Sub
2. Create a Sub inside your Second Form to call the Private OnUpdate of your second form.
'Form1's Module
Sub mySub()
doCmd.OpenForm "Form2"
Forms.Form2.runOnUpdate
End Sub
'Form2's Module
Sub runOnUpdate()
Form_OnUpdate
End Sub
Private Sub Form_OnUpdate()
'Your Code here
End Sub
I would suggest using the second option.
Of course, I'm not even sure I understood your question so this could be of no help to you whatsoever.
Peace