>>17682206The only way two 5IV Pokemon with the same spread will generate a 6IV is if the Destiny Knot chooses the missing IV to skip and generates a natural 31. So, that's 1/6 x 1/32, or 1/192. Any of the other 5/6 times, it will pick one of their bad IVs and ruin the baby.
If both parents are missing different IVs, it means that it still has a 1/2 chance of picking the IV the one parent has that the other is missing. So, 1/6 becomes 1/3. 1/3 x 1/31 is much better odds.