If you happen to have a 4P4C crimping tool around, it's very easy to make a longer cable, which is what I did. If you don't, then at least I found it pretty difficult to find a 4P4C crossover cable in Finland. But it should be findable online.

I was under the impression that UHK uses a common "phone cable", at least common here in the north?

Only one pair is required for a single-line telephone installation, and at least here in the US, most cheap 4P4C cables thus omit the second pair - especially now that landlines in general are so much a thing of the past. I don't know whether the UHK will work with a single-pair cable - I suppose it might, if it uses SPI or I2C or 1-Wire for signaling between the halves, but I'd definitely check the manual before just slapping any old phone cord in and expecting it to work.

Common phone cable in Finland seems to be 6P4C, at least the ones I had.

Any (most?) phone chords work for this. I wanted a longer one and it made for a good laugh when I asked my partner for one as a gift.

That is interesting. Can you post a pic of how your keyboards are placed?

Two 60% keyboards side-by-side, no gap between them. My forearms stay parallel and my wrists straight. It is also good to confuse people into thinking you have the especialy hability of typing in two keyboards, when it is not much different than touch typing in a single keyboard. Also it's a good excuse to buy another mechanical keyboard.