Our MVVM sample, which is included in our Sample Browser, does not support that type of "relative" position. Our Prism Interop and samples do support a DefaultDockRelativeGroup. So you can create two groups, and say group 2 should be docked below group 1. As layouts get more complex it gets much harder to "describe" that using simple configurable properties like DefaultDock, etc.
In both our MVVM sample and our Prism support, you have full control over where things are opened. In MVVM, you could just tweak the DockSiteViewModelBehavior class to fit your needs. In Prism, you can create a custom ToolWindowInitializer and override it's Open method.
In both cases you can perform any type of programmatic layout. But what you are describing is not possible with the DockGroup and DefaultDock.