(please also read http://www.plugcomputer.org/plugforum/index.php?topic=6238
!!! SUCCESS !!!
There might be a more straight forward way, but here's how it worked for me
As mentioned in the other thread I amended the "2.08.28 WHQL Certified" INF files with the VID/PID information of the ones in the INF files of the "CDM 2.04.16_SHEEVA" version.
I then disabled the driver signature enforcement in Windows 8 (see http://www.craftedge.com/tutorials/driver_install_windows8/driver_install_win8.html
After that I manually directed W8 to update the driver from the modified "2.08.28 WHQL Certified" directory for the two unknown devices.
Win complained about the signature, but I confirmed to use that driver.
However the "USB Converter Serial A & B" showed up properly in the HW manager, the newly created COM port was flagged with a failure.
I couldn't get that fixed so I removed the devices and drivers again.
With my 'second' try I picked the "CDM 2.04.16_SHEEVA" drivers (again with disabled the driver signature enforcement).
Again both "USB Converter Serial A & B" showed up, but again an new USB COM port was flagged with a failure.
I picked this device and applied the "2.08.28 WHQL Certified" driver to it and EUREKA, COM9 'arrives'.
W8 reports 18.104.22.168 as the driver version for all the devices.