The signal is effectively cut in half. One sub will play one half of the signal and the other will only play the other half, but since they are in different locations, they will be out of phase, and the signal will suffer self-cancellation which culminates in data loss.
Oh, also at the separating juncture of the split, electromagnetic "tear" discharges energy which saps the upper bounds of the signal strength, which thusly results in compression, and again, you have a loss of data.
I should also mention low voltage backflow, when, if you have two subwoofers on a single cable, can create a reverse short circuit thereby producing a data inversion- a form of data loss.
And, of course, the magnetic field flux from the woofers can 'leach' on to the cable's magnetic field, which is normal, but if you have two opposing woofer fields, the recombined 'leached' energy carry can cause total protonic reversal. Needless to say, that will cause data loss.