Here is a simple example of how to do what you are asking using 2 maps and 2 Legends.
<Grid x:Name="LayoutRoot" Background="White">
<esri:Map x:Name="Map1" Extent="-20037507.0671618,-20037507.0671618,20037507.0671618,20037507.0671619">
<esri:ArcGISTiledMapServiceLayer ID="basemap" DisplayName="Base Map 1" Url="http://services.arcgisonline.com/arcgis/rest/services/world_topo_map/MapServer" />
<esri:FeatureLayer ID="points" DisplayName="Incident Points" Url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/HomelandSecurity/operations/FeatureServer/0" DisableClientCaching="True"/>
</esri:Map>
<esri:Map x:Name="Map2" Extent="-20037507.0671618,-20037507.0671618,20037507.0671618,20037507.0671619" Visibility="Collapsed">
<esri:ArcGISTiledMapServiceLayer ID="basemap" DisplayName="Base Map 2" Url="http://services.arcgisonline.com/arcgis/rest/services/world_topo_map/MapServer" />
<esri:FeatureLayer ID="lines" DisplayName="Incident Lines" Url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/HomelandSecurity/operations/FeatureServer/1" DisableClientCaching="True"/>
<esri:FeatureLayer ID="polygons" DisplayName="Incident Polygons" Url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/HomelandSecurity/operations/FeatureServer/2" DisableClientCaching="True"/>
</esri:Map>
<Grid VerticalAlignment="Bottom" HorizontalAlignment="Right">
<esri:Legend x:Name="Legend1" Map="{Binding ElementName=Map1}" LayerIDs="points" />
<esri:Legend x:Name="Legend2" Map="{Binding ElementName=Map2}" LayerIDs="lines,polygons" Visibility="Collapsed" />
</Grid>
<StackPanel VerticalAlignment="Top" HorizontalAlignment="Right" Margin="10">
<RadioButton GroupName="MapSwitcher" Tag="1" Content="Display Map 1" IsChecked="True" Click="RadioButton_Click" />
<RadioButton GroupName="MapSwitcher" Tag="2" Content="Display Map 2" IsChecked="False" Click="RadioButton_Click" />
</StackPanel>
</Grid>
private void RadioButton_Click(object sender, RoutedEventArgs e)
{
string tag = (sender as RadioButton).Tag as string;
switch (tag)
{
case "1":
Map1.Visibility = Visibility.Visible;
Legend1.Visibility = Visibility.Visible;
Map2.Visibility = Visibility.Collapsed;
Legend2.Visibility = Visibility.Collapsed;
break;
case "2":
Map2.Visibility = Visibility.Visible;
Legend2.Visibility = Visibility.Visible;
Map1.Visibility = Visibility.Collapsed;
Legend1.Visibility = Visibility.Collapsed;
break;
}
}
}
This should give you the functionality you are looking for and you can then change it to meet your needs.