replace user control in grid

replace user control in grid

My xaml looks like this:
<UserControl>
<Grid>
...
<Grid>
...
<Grid Grid.Row="3">
<local:UserControl1 Visibility = "{Binding Control1Visibility}" />
<local:UserControl2 Visibility = "{Binding Control2Visibility}" />
</Grid>
</UserControl>
Control1Visibility and Control2Visibility are properties in my ViewModel,
but it isn't work. This grid couldn't dynamically display the two user
control. Anyone has ideas?