if the one cable is fine in both tuners, and the other cable is dead in both tuners, then you have used whats known as the 'split method' process of fault finding, you have shown using one cable that both tuners are fine, the other cable is dead, this points to either that cable, the connections on that cable, or the LNB (if its a twin output lnb)
assuming you have a single fixed dish outside, plug BOTH cables back in, and take a note of which tuner shows a signal, go to the dish, and swap the connections around on the LNB, if you have a quad LNB, ensure you use the same 2 connections for now
if tuner1 had a signal, and still has a signal after swapping the cables around, then its a problem with the cable (or the connections on the cable) thats plugged into the input marked tuner2
if tuner 1 had a signal, and after swapping the cables around tuner2 has a signal, then its a problem with the LNB, if you have a quad LNB, then use the free outputs and youll be back up and running (assuming theyre ok)