You basically can define a region everywhere. Define a Region for a View and withhin this View you can defina another Region.
<!--Main Window-->
xmlns:prism="http://prismlibrary.com/"
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Content Grid.Column="0" prism:RegionManager.RegionName="MainRegion1"/> 'Injects View1
<Content Grid.Column="1" prism:RegionManager.RegionName="MainRegion2"/>
<Grid>
<!--View1 in MainRegion1-->
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Content Grid.Column="0" prism:RegionManager.RegionName="SubRegion1"/>
<Content Grid.Column="1" prism:RegionManager.RegionName="SubRegion2"/>
<Grid>