These are not made up sizes. They are standard paper sizes available in any store. Most ordinary househole printers won’t actually print on tabloid, and some won’t print on memo, but all copiers (and multifunction scanner/printers) can copy two letter pages and print them side by side, scaled down to memo size, on a single sheet letter of paper. It’s just a button you press, or maybe a checkbox you check, or whatever. It’s not magic.

The sizes aren't what I said you'd made up. The series is, because of course this doesn't work properly with your scheme:

Let's try, start with 216 x 279 Letter and we'll scale that so we can fit two on the 279 long side, so they're just under 140mm wide now, memo size, but um, when we do that the scaled down pages are only 180mm high, even though they've got 216mm - there's a huge amount of empty white space wasted (at the top, or the bottom, or both) and that's because this ratio does not work.