http://www.dds6qkxpwdeubwucdiaord2xgbbeyds25rbsgr73tbfpqpt4a6vjwsyd.onion/w/index.php?title=Dev/Qubes&curid=1376&oldid=83635
This is how other VMs in Qubes are networked, including TorVM. According to Qubes devs, these Xen backends provide simple point-to-point networking between VMs. So this would be advantageous for further isolation of inter-VM Whonix traffic, since, currently, all inter-VM traffic is routed through the internet-facing FirewallVM, which can also be shared by other VMs.