"Undo" biwiring?

BTW I am talking about combing connections between internal wires on the inside of the cabinet, in case that wasn't clear...

Perfectly OK to put both wires on one post. (If they were my speakers i would make a label mentioning the second set are disabled.)

Instead of soldering, why not simply use the same wires as jumpers? I do not understand why you think that "expensive jumpers" are the only alternatives to an internal intervention.

You mention that you've tried using jumpers. Did you mean to say "which have not made a difference..."? The reason I ask is that jumpers don't provide the benefit that biwiring can.

If the high-pass crossover functions by presenting a high impedance at bass frequencies (as they normally do), then with biwiring to the high-pass connection, little to no bass current will flow because of the high impedance at low frequencies.

This means that few or no magnetic fields would be generated by the (now absent) low frequency current which could otherwise modulate the midrange/treble. This should make your midrange and (especially) the treble sound cleaner. Using jumpers or a single connection doesn't provide that benefit since the bass, midrange, and treble current all flow through the same cable up to where the jumpers connect or to where the low-pass and high-pass leads are internally joined.

When I biamp my system, I use a less expensive speaker cable from the bass amp and use the high performance speaker cable on the top, though ideally the type of cables would best be used on top and bottom, so this is a cost compromise. When I biwire using one amplifier's output, I biwire with the much better sounding, expensive speaker cable.