Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

At this point it's moot, but do Klipschorns go lower than the Cornwalls.  If so, get a pair of of the corner horns and only run the woofer sections.  This setup would provide excellent matching of dynamics, speed and timbre.

I would NOT add subs to those Cornwalls, with 15" woofers, ports, and high efficiency horns.

You could easily mess up what you have, it’s very hard to add just the lowest ...... down to ....

You are looking at specs that say down to 34hz. that is in a test space, open air, your’s are in a room, sound waves contained, reflected, your bass is maintained below 34 I suspect.

I strongly suggest you buy a decent sound meter, see what you are actually getting, and where. Look for any weak/too strong spots, standing or cancelling waves.

With that sound meter, if you add subs, (or change speaker placement) you will be able to measure/adjust the crossover/blend from your listening spot.

2 channel system: 1 pair of subs, directional, located next to the cornwalls, forward facing. Bass is stereo, and the overtones of all low frequencies also give directional cues.

+10
Adding a sub to the Cornwall will be an exercise in frustration. Very little program exists below 30Hz, so work on placement. Depending on the room, REW’s room simulator [https://roomeqwizard.com] or Loudspeakers Calculator at Dr J. Hunecke’s Room Acoustics can assist in determining optimum location

I ordered the Hsu VTF-15 II yesterday.

you'll be sorry...
Get REW and a microphone and discover the bottom end phase response is a bucket of sludge.

Since you gone and done it, please read  http://www.ielogical.com/Audio/SubTerrBlues.php for some help on setting up. Read the links as well.
Bonne chance!

congrats... and,you can a second one later on when budget presents itself

neither subs are a waste of Money.

Go with the two Hsus (but depending on your main speakers, consider the  VTF-15H MK2 as well - goes lower and has a bit more output. Maybe better if your main speakers have decent bass and you want to cross over lower down.