The dish LNB needs to be a DPP twin to feed two duo receivers. If it currently says DP or nothing on the LNB, then it needs to be changed out, or if it has four outputs (quad), you could run two more coaxes instead. If you change to a DPP LNB, then the DPP seperators that come with the receivers will split the signal to each tuner.
Don't confuse TV2 with tuner 2, duo receivers have tuner 1 and 2 inputs, each needs a connection to a dish. They also have TV2 output which can feed additional TVs with a coax. A pair of diplexers can insert the TV2 signal into the dish coax and take it back out to utilize a single coax for both the dish signal and TV2 signal (backfeed). Better to run another coax if possible.