This is my most investors seem to choose to set up additional LLCs later as they grow, so they create firewalls between their assets: if one burns down they don't loose the other ones.PS: this reflects my personal findings - I am not a lawyer and not your advisor.

The units were side-by-side with a firewall in between them. 

That means most likely, a fire wall dividing the two units, all the way up to the roof.

They typically have requirements like firewalls to install, what constitutes a kitchen, etc.

I checked with Mecklenburg County Code Enforcement and they said the units would I would have to have fire walls between the living spaces and would have to submit drawings before I had the work done.  

Like Matthew pointed out you would likely want to consult a local attorney, but in my area there is little to no downside and increased anonymity and firewall effect by utilizing that strategy.

That is not a condo and it would require certain rehab features, like firewalls, don't even go there.

You probably want to make sure it is up to code, ie. firewall , etc...check for permits pulled. 

Way I see an LLC now is an illusive firewall between me and a lawsuit, but that may change as I learn more.

From some other advice I have received, one of my biggest costs would be installing fire walls between the units.