I'm very interested in buying a duplex and living in one space and renting out the other.

Your share will be determined by the amount of space you rent in relation to the overall building size.  

Shortly after they moved in, the downstairs tenant asked if she could install hookups in the basement, for her use, to free up space in her kitchen.  

Particularly when there is no space to add a 2nd bathroom on the 1st floor and a laundry room is needed.  

How easy or common is it for a commercial real estate owner to decide to split the property to rent out to both commercial and residential tenants if they aren't able to rent out the whole building as a commercial space?

In my space, no one is doing that and it differentiates me, allowing me to charge above the market.

He was told by his accountant 2nd floor space is cheaper then first floor space so the 4 and change he gets downstairs he would not be able to get.

Do you prefer to place the stove in the middle of the counter-top space to breakout the counter space?

Space isn't an issue and probably have more land the I can afford to build on.I guess my question is, is there a sweet spot for number of units?

ok sister 2 2x4s to an adjoining stud and then sister 2 2x8 across for support. that sounds very sturdy for the small space that I am going thanks.