I cannot see how build two, say, 1200 sq ft houses with two roofs, two foundations, presumable about 25-30% more windows and siding is going to be cheaper a duplex that might have a little extra fireblocking and soundproofing.Now, maybe your GC is thinking that detached homes tend to get higher rent than comparably sized and finished duplexes.  

Don I have spoke at many of these events.. the one here in Oregon though I personally dont care for much.. they tend to bring in old retread type guys that are pitching systems that worked 25 years ago.. and I pretty much believe they are taking kick backs for the sales those guys are making..

This is because I recommend at least 20% down and the homes tend to cost $500k-$650k for the lower-end budget. 

Also, air conditioning removes moisture/humidity from the interior airspace, and nearly every home in the Carolinas runs AC during the warm months.Crawlspaces tend to be where the majority of problems occur.

They tend to take better care of the property.

In my experience people ho have no skin in the game tend to care less about keeping the property up than those that have to work to have it.

Same goes for number of LLCs and what to fund them with, since bear in mind that CA tends to be more cumbersome and expensive to have LLCs than other states.California is generally more cumbersome than other states when it comes to taxes and filings.

Fans tend to last a very long time.

In addition, multifamily homes tend to appreciate quicer as well. 

I've noticed in my area that houses that have been converted into 2 units tend to sit on the market for much longer and sell for much less than traditional single family houses with similar sqft and finishes.