At the center is a rough custom Behavior that is able to share the visual state of controls that are bound to the same data object. Once the behavior was functional, I simply needed to add the behavior to two Buttons, bind the buttons to the same data, and wire an event to trigger the state change.

<Button MouseEnter="enterHandler" MouseLeave="leaveHandler">
  <i:Interaction.Behaviors>
    <local:SharedVisualStateBehavior Source="{Binding}" />
  </i:Interaction.Behaviors>
</Button>

private void enterHandler(object sender, MouseEventArgs e)
{
  SharedVisualStateBehavior.GoToSharedState(
    (Control)sender, "MouseOver");
}
private void leaveHandler(object sender, MouseEventArgs e)
{
  SharedVisualStateBehavior.GoToSharedState(
    (Control)sender, "Normal");
}

Because of the need to call a static method to actually make use of it, meaning it’s not selfcontained, it doesn’t really qualify as a Behavior.  The ideal SharedVSM would be able to take input in the form of VisualState parameters and possibly a trigger, in order to notify the shared controls of state changes.  It’s a work in progress.

Thanks to Kuler for the theme, Josh Smith for ObserveableObject from MVVM Foundation, and Pete Blois for BindingListener from Expression Samples.