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How does the Bis effect work, and do I really write two numbers in one turn?
The designer described the procedure in 2019 and 2020, matching the rulebook's Bis section.
Yes, two houses. First write the number on your chosen card in any street, respecting ascending order. Then, if you use Bis, pick any already-written house on any street (including the one you just wrote) and write the same number with "bis" in an empty house directly next to it, to its left or right. The copied number does not have to match the card.