Quite simple, with a built in Hard Drive.
The Duo 2 has two card tuner slots, each tuner card can have 2 satellite tuners, thus 4 separate tuners, which need to be fed by 4 coax cables, either from a quad LNB, several fixed dishes, pointing at various satellites, or any other dish/LNB set up.
Thus you have Tuners A, B, C and D.
Each tuner works like an individual receiver, so you can record 4 different satellite transponders, each of which can broadcast a number of channels, generally 8-10, but can be more, or less. Once you start recording a program, on a transponder, on Tuner A, you can still view/record, other channels, using that transponder, on Tuner A, the same for B, C, and D, providing more than the 16 mentioned, which might be a software/harware limit.
Add to that you can install USB terrestrial tuners, or cable/terrestrial tuner card, and the options are too varied to go into.