food for thought:
While you do seem to be going in a different direction, let me answer the original questions.
- dual output C-band only LNBFs are available from SatelliteAV (
GeoSatPro C2), Sadoun (
GeoSatPro C2), and Galaxy (
DMX242).
They're quite inexpensive, and easy to experiment with.
- there are still some Pansat receiver models which will drive a corotor servo.
I believe the 2700 ? and the 3500sd will do that.
Both are standard def only, and while not obsolete, are certainly not what most of us want for the future.
- the 12/18 volt servo interface box was described quite well on this forum some time ago.
Building one wouldn't be too difficult.
However, it would not output both H & V polarity at the same time, should that be required, and would continue to rely on older servo technology.
- a suitable Ku feedhorn might be a challenge.
I would probably think first of the AF-120 from Invacom, and marry it to any of their flange-mount, dual output LNBf's.
The QPF-031 comes to mind, if somewhat expensive.
If there is a way to convert a less expensive two-output Ku LNBF to the AF-120, I haven't thought of it.
Oh, wait, the old Primestar LNBs with H & V output might work.

Well, that's an area for some thought.
Bandstacked Approach:
This is not for everyone, but I do bring it up just to offer it for consideration.
The Ku LNB could be a bandstacked Eagle Aspen or DishNetwork FSS device.
It mounts directly to the AF-120 feedhorn above.
The C-band LNBF could be an Eagle Aspen
B1 Sat stacked device.
It comes with a suitable scalar ring.
Previously discussed on the forum, a DishNetwork DP-34 multiple-output diseqc switch suports three LNBs and outputs to four receivers.