Just this year, one of the houses had a collapsing septic tank which I had to have removed as the backyard was getting a sinkhole making it a danger to the kids playing. 

In the mean time, I removed the LP heater and the leased LP tanks, so there is no more gas used in the place (so the CO part of the alarm shouldn't be an issue) and installed a mini-split heat pump in its place.I hadn't heard anything from the tenant but, when they moved out last week, I saw that they had again disconnected the alarm in the bedroom.

You need electricity, a sewer disposal tank, and 24/7 lighting, and portapotties 

The sewage from unit A and B fall into a shared sewage tank. 

Lease price, structure, length and longevity of the current tenantsCondition of the capex items including the roof, structure, water tank, furnace, ac, appliances etc.Their experience with that zipcode/pocket if they are an active realtor

Let's say you get a service call for no hot water when it gets checked out there's no gas to the hot water tank to light the pilot.

On the 11/27/17 podcast he clearly explains how if you have $400k in the bank and take it all and pay off your house and then the market tanks and your house is now only worth $200k, you just lost $200k in a second.

The toilet vale in the tank is a good idea.

What are the pros & cons of having water tanks shared on a mutli family unit?

They just get too cute and swim as minnows in a pool with sharks and whales.