If the structure qualifies for a partial disposition, create two new assets, splitting the basis and prior depreciation proportionately (probably 50-50 in this case), and then delete the old asset. The goal is to create a situation in which if all you did was split the old asset and then deleted it, the two new assets (top unit and bottom unit) would generate the same depreciation (together) that the original asset generated.
Once you have done this, then dispose of the split asset that you assigned to the bottom unit. Of course, you also have to create a new asset to represent the new bottom unit.
