Actually, there are three components to common law marriage:
First, you must have "agreed to be married."
Second, you must have "held yourselves out" as husband and wife. You must have represented to others that you were married to each other. As an example of this, you may have introduced you partner socially as "my husband," or you may have filed a joint income tax return.
Third, you must have lived together in this state as husband and wife.
Other than living together, it all comes down to what you say and how you act. If you were facing legal challenges, you could easily "prove" that you were common law spouses. I expect that would insulate you from the unrelated persons code, but I can't guarantee it.
As for houses zoned for multi-family, I have never seen such a thing. It may exist, but I have never seen it. The closest I can think of are the multi-family homes that are common in many cities, where there are two or three complete residences in one house - one on each floor. These are no different than duplexes though, except for the layout.
Still, there is nothing preventing such a thing in San Marcos. Anyone could buy multi-family zoned property, build some houses and rent them out as you describe. The city can't force people to do that and I am not sure how they could encourage it.
If you talk to the owners of the properties on Sagewood, they will tell you that they stopped maintaining them when the residents kept trashing them. It cost too much to repair them every year, so they stopped bothering with them. What is is about a house that makes it any less likely to suffer the same fate, whether those duplexes were trashed by the tenants, neglected by the owners, or both?