Looking at the schematics (
http://sportsbil.com/tandberg/3000x-s.pdf), the headphone output is actually wired in parallel with the radio output so it shouldn't matter which one you use. (These particular schematics are for the US version of the machine which has Phono rather than DIN connectors).
Remember too that on the Tandbergs, when you only press one of the tape playback buttons, the corresponding channel (L or R) is sent to both the L and R outputs.
Regarding special cables, when copying between two decks, a DIN cable with the record and playback signals swapped over would be needed, in contrast to connecting to a radio or amplifier where a straight through cable is used. So the "special" cable designed for tape copy is just a signal routing thing and has nothing to do with the signals themselves. In the OP's case, once you have the two playback and to record signals available on separate connectors you have got all you need.