an internal biwire normally has 4 wires per channel in a common sheath - very little physical space between HF and LF conductors
principal theory being the large current demands of LF and the field that generates modulates the HF signal = degredation
a shotgun typically has common termination at amp ends but splits the HF and LF runs into different cables that importantly can then be spaced 3-4” apart
you can kind of get same effect w two sets of cables, i personally dislike stacking spades and trying to get a gas tight connection and not get shorts at the amp end, hence my preference for shotgun
hope that helps