I've gone through the LinuxDVB API description regarding the frontend again and apparently it is documented that multiple frontends on the same adapter can't be open in read/write mode at the same time (so the "deficiency" is actually on VDR's side ;-).
Well, so VDR could open them in read-only mode first and switch one of them to read/write mode shortly whenever it does a tuning operation, and go back to read-only after that. It would also have to switch to read/write shortly whenever it reads a frontend event with ioctl(FE_GET_EVENT). With such modifications it should be possible to make VDR support a multiple frontend adapter. In order to set up the necessary VDR devices, cDvbDevice::Initialize() would also have to be enhanced to probe for multiple frontends on the same adapter.
I'm very happy if you could try to make such patch.
I can test that patch, and make few adjustment, since I think you can't really test it.
Thank you for your support
Best Regards,
Eddi