To guarantee ascending order in both columns we can compare by column when there is no `nil` and by max value when there is:

    .sort { |(a0, a1), (b0, b1)|
      if [a0, b0].none?(&:nil?)
        a0 <=> b0
      elsif [a1, b1].none?(&:nil?)
        a1 <=> b1
      else
        a0, a1, b0, b1 = [a0, a1, b0, b1].map(&:to_i)
        [[a0, a1].max, a0, a1] <=> [[b0, b1].max, b0, b1]
      end
    }

1. Compare two items at a time
2. If the left columns are both positive integers then use that to sort.
  * this works because if they both have numbers then the right number is always correct relative to the left, so sorting on the left gets you the right result. 
3. If the right columns are both positive integers then use that to sort.
  * we know it's not a matched pair and both left numbers are nil and both rights are comparable so...
4. Convert all the nils to something we can work with (`.to_i` makes them zero).
5. Otherwise, create arrays to sort, with the 1st element of each array being the non-null (non-zero at this point) number. `.max` is just an easy way of getting the non-zero one.