Hi crazycat
It's hard to argue with the developer

, I was just refering to the manual for the 6508 where it says that it is only possible to send diseqc messages on the master tuner. So I did some testing in tvheadend and basically the driver accepts the diseqc message but it seems to never arrive at the lnb. What I did was the following:
Enable only tuner #0 and make sure everything is working as expected.
Disable tuner #0 and enable #1, now I can tune normally on the last transponder that was used with tuner #0 but everything else won't lock. I can also add a transponder with the same basic settings (SR/FEC,System...) and a completely different frequency and it will still scan the same transponder which was last used on tuner #0.
I also enabled debug/traces in tvh and made sure it does send the correct messages to the device (at least from tvheadends point of view). So although it seems everything is perfectly right, nothing seems to arrive at the output. I don't know how much you are involved in the firmware developement, but it might be discarded just there.