For your example scenarios. I dont see any real reason why 1 person could not own both units in a 2 unit building when both are condo's.
As for "rules", different lenders have different guidelines.Some are laws, some lender standards and some are specific to the lender and the risk level they want to take.
Does it become more difficult to find financing if the owner ratio is low ? Yes it can, but there is usually a lender who will consider it, but in a small building this can usually be resolved in my experience.
Call me for a referral