What kind of
DTV system do you have? Ie a single sat system or a multi-sat system? I'd think that you could get it to work in somewhat of a clunky way, if you have a simple 1 sat system.
However, I'm confused about how you have things connected back at the receiver end of things. Ie how do you have 2 receivers connected to one coax???? If you've used a "T", then you must have one of the 2 receivers controlling things (and have a DC-block to the other receiver), and the DTV receiver can't control things because you don't really have control over DiseqC commands with it. Also, with a "T", you lose 50% of your signal strength.
If you really have to do this, I'd recommend having your DVB receiver connected to the coax, and program one satellite to use the LNB2 input of the
diseqC switch, and furthermore, program 2 channels, one which is horizontal, and one which is vertical. Then, attach a DC-block to the passthru of your DVB receiver, and connect the DTV receiver to that. You can then tune the DTV dish with the DTV receiver, HOWEVER, you will have to change polarity via the DVB receiver.
I have done this, and it works, however, it is VERY cumbersome, and you'd be much better off running a second coax. There is ALWAYS a way to snake in a 2nd coax.