Yes, both servers are turned on, and I can access them reliably using DLNA clients on the PC and two iPads.
The discovery process is very flaky on both HDRs. The PC is the only other DLNA server, and can normally be seen from both HDRs, after a lengthy search, however sometimes it disappears. When this happens it seems to be necessary to use the Portal, or visit the Menu Internet settings to restore the connection.
Also, if storage is set to Photos, I have only been able to access pictures on the PC just twice, out of perhaps 20 attempts.
The CF sharing solution seems to be the way to go, thanks for the pointer. However, I'm not entirely happy with my network, and it would be interesting to know if you or anyone else out there has two HDR's which can stream to each other using the standard DLNA server/client.