purchasing a duplex along with a friend (selling and property value)

Hi
My friend and I (both married) are thinking of purchasing a duplex. The
intent is to divide the lot if needed and sell it independently later.
This seems to be possible legally subject to each separate unit meeting
minimum conditions for being a separate house.
However, we are not clear on
a) if there are any issues with the value of the each unit being lower
than for a comparable house eg. if we purchase a duplex with 2 bedroom
1.5 bath unit built side by side - will the value of each unit be lower
than as compared to 2 bedroom 1.5 bath condo? Are there any general
rule of thumb in this regard

b) Is there an source of legal info on how such units are sold? What
kind of agreements do we have to have between ourselves?
Thanks for any answers