I would have entered the original asset at 50% of basis used 100% for business.  For real estate the math ends up in the same place.  In your case I would consider cutting the basis in half and changing business use to 100% on the old rental asset.

Either way, then I would add a new asset for the other half of the unit.  Depreciable basis for the new unit is the lower of cost basis (50% of total if they are comparable) or FMV on date of conversion from personal property (in most cases that's original cost).  Use the date placed in service and this new asset will depreciate over its own 27.5 year period.  There may be other assets placed in service as well (improvements, appliances, etc.)

I would even consider putting this in a different Sch E column.  Makes it easier to unravel if/when the owner moves back in to one of the units later.

Rick