Most houses are wired with 2, 3, or 4 pair cable (since the 1980s.) Before that, "Quad" wire (4 conductors) was used (since the 1960s). Before that, Bell used 3 twisted wires.
Quad wire can be used to carry two phone lines; Bell used to do it, though it was discouraged. The original purpose of the wires was Red + Green for the voice pair, and yellow for ground. In some phone lines (certain party lines) an independent ground was needed. For traditional single-party residential lines, only two wires (red+green) have been used since the 1970s.
The yellow+black wires in Quad cable were often used to power the lights in Trimline and Princess phones. But local Bell craftspeople, perhaps lazy, would use the yellow+black for a second phone line. The problem with doing so is that you can get cross-talk between the two phone lines.
Since the 1980s, twisted-pair wire has been common. In these cables, you have pairs of wire twisted together, keeping them from interfering with other pairs. Typically, the first pair is White with blue stripe + Blue with white stripe (WH/BU + BU/WH) The second pair uses white and orage (WH/OR + OR/WH), then green, etc.
If you have that kind of wire running through your house, you can use the 2nd pair to hook up the second phone line.
Lots of 2-line phones these days happily plug into modular jacks where the first line gets the center 2 pins on the jack, and the second line gets the next outer pins.
Radio Shack used to have books describing home phone wiring, and they certainly have all the parts. Or, tell us more about what you have and we can advise.