I got the plastic filter wrenches. Attempting to use the wrenches did absolutely nothing. I'm pretty sure I heard the filters laughing at me - a cold, dry laugh.
I tried heat and cold - ice from one side and hot aluminum surface from the other. No success.
I left the filters in water overnight. Nada.
Got desperate...
WD-40 in the threads, let sit overnight. Nothing. Repeated a second night - nothing.
Finally I used a tool to gently pry the filters apart. I worked around the circumference, and at some point the filters popped apart enough so I could unscrew.
Threads seem undamaged but some brassing now shows on the front of one filter.
Next I used dishwashing detergent to remove the WD-40 and clean the filters - glass was spotless, zero damage.
I was hoping to sell these filters but obviously they are far from mint in appearance anymore so might as well keep them.
I figure that what happened was that these slim-line filters has so little thread that the upper filter got all the way down to the retainer ring for the glass of the lower filter, which offered far too much friction and simply locked. Manually slowly threading them back together sort of confirmed this, there was definitely a point of locking up again although I was careful not to tighten at all.
Thanks everyone for practical and creative ideas!
Lars