Are you saying that you want to create two users controls using one button click?
I create my own example using your code and it worked fine. I used two panels, two user controls, and a button. By the look of your code I assumed frmMain to be a reference to your main form passed to a second form where the button click handler was located so I did this as well. Here is my code and it worked just fine for me if it is not working for you I assume you are just overlooking some small detail.
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim obj1 As New UserControl1
obj1.AutoSize = True
obj1.AutoScroll = True
frmMain.Panel1.Controls.Add(obj1)
Dim obj2 As New UserControl2
obj2.AutoSize = True
obj2.AutoScroll = True
frmMain.Panel2.Controls.Add(obj2)
End Sub
Let me know.
Also if you are getting any errors please post them.