
my first litter only had 1 male and I felt them both down at less than 6 weeks, they were only the size of apple seeds, next litter, same bitch different stud had 4 pups all male and they varied between 6-9 weeks before both were reliably staying down.
as they start off above the kidneys,one higher than the other they have some travelling to do and unfortunately the gap they need to pass through closes before the second one has descended.
my parents first mini long dachsie only had one and the other one was missing altogether, he lived till the age of 13 with just one and had no problems.