Thank you for posting in the forums! I'm sorry to hear you are having trouble using the dual tuner with your DirecTV set-top box. We do have a article in our support site that may help you resolve this issue.
Please, take a look at the article below:
Feel free to reply!
The Sling Moderation Team
As far as I can see from images of this device it has only outputs on the back for the one primary tuner ? Most devices like this simply use the multiple secondary tuners internally to record programs on the Hard Drive and do not give access to the user.
Are you able to plug in more than one TV to watch different programs ? If not you cannot expect the Slingbox to access any of the secondary tuners.