Bi-wiring with different cables for HF and LF?

Not a problem, why not order another pair of Morrow SP-2 on their trial, then use your broken in pair on the LF. I wouldn't be overly concerned about gauge. A couple of years ago, I used Audience Au24, a very small cable, and the bass was extremely good.

I've been bi-wiring and bi-amping for years using different cables/wires and find it a very useful tool for achieving the sound I want. Some have argued against it.....using wire as a tone control but who cares if it sounds good? You can certainly get mismatches which will sound odd if not outright bad but only experimentation will tell. For what it's worth, I found silver ribbon/foil wire to have the best bass definition and slam. Electraglide silver ribbon speaker cable (Goertz AG-2 I think) really took my 4 12" Peerless 1259 bass drivers to the next level in articulation, clearly placing bass instruments in space, with great clarity (Stanley Clarke was nearly in the room with me), and bass attack could be quite startling.

If you want to experiment on the cheap, try some wire from Ebay. I made some power cords using the Mundorf wire from Taiwan (is it real Mundorf? Don't know, don't care---it sounds very nice and is inexpensive)and they easily compete with higher end manufactured power cords (Wolff, Sunny Cables, TG Audio, BMI). Get some heavy gauge mil-spec silver plated copper and try them on your bass drivers. I highly recommend NOT using any connectors. Simply connect bare wire to both the amp and speaker terminals for the purist connection. I would avoid gold plated wire though as, in my experience, it tends to loose detail in the lower registers and trades both articulation and slam for weight. YMMV. Good luck.

You can certainly try it without causing any harm. Let your ears tell you if it what you are looking for.

The idea that you can't use different cables for LF/HF is BS. Use whatever sounds good to you. I use different cables for them and it works. There is no dogma.

All wires have a particular voice. By using different cables you are voiceing the high section differently than the lows. Coherence is compromised. However...you can do what you want...no damage