This is all a perfect example of why you should always use a DC-block on the pass-thru of these DVB receivers. The analog receiver, or a second FTA receiver on horizontal, will put out 18V, so that depending upon to what extent the first receiver isolates the pass-thru, if at all, when the first DVB receiver tries to go to vertical at 13V, the analog or 2nd DVB receiver can over-ride the 13V and send 18V to the LNB. It's also very possible that the 18V can get into the first receiver causing damage. And if the 2nd receiver happens to be putting out 13V while the 1st receiver is putting out 18V, again depending upon how the first receiver is designed, it's possible to send voltage back into the 2nd receiver.
Although I've seen many people say they use passthrus without DC blocks, it simply isn't worth the risk.
I also no longer trust those splitters with the one DC-pass leg and the other ports supposedly protected by DC-blocks. I recently posted about issues I've had with one of these as well, in that it didn't isolate voltage between receivers that were supposed to be protected by the internal DC blocks.
Anyway, I like to always use DC blocks, unless I am certain that the splitters I'm using are really working.