@George Wines, I'll chime in here, but @Mike Wood and @Jeff B. are right on.
As a former city planner in the Front Range, I can think of several legal issues preventing you from condominiumizing a duplex. There's usually a party wall agreement, instead of CC&Rs that outline a commonly-held parcel with footprint or air rights to the units themselves. A duplex condo would be notoriously difficult to mitigate disputes between two parties (there is no majority vote).
You'll also be looking at separating water service (two direct lines and associated extra tap fee instead of one line that splits) and bringing the party wall up to fire code (2 hour separation throughout, even in the attic space). Not to mention, the minimum lot frontages, lot sizes, or access to a main street (not alley) are hard to meet.
It's a nightmare for future owners, and frankly, I've seen very few duplexes legally separated.
