Thoughts on appraisal/valuation of two apts in 2-fam house?

My wife & I are buying a two-family frame house near the 9th st f/m/r station. Our friends are buying it with us, as there are two duplexes in the 4-story house. The bottom duplex has about 150 more square feet, but also exclusive access to the garden and basement.

We are trying to figure out the most equitable way to value each apt so as to split the mortgage (we can't get separate mortgages until we condo the house after we buy it). Does anyone have any thoughts on whether the garden should be valued simply by sq footage or also potential investment/resale value? Info from anyone who has been involved in such situations or has thoughts on how best to proceed would be much appreciated. Thanks, Alex