What might banks think about offering an investment loan to two partners, one with decent steady income who just signs the mortgage and one who supplies the down payment? 

I have been doing Airbnb solely for 2 1/2 years an a SFH and even with internet connected lock and camera, maid service, auto price online service; I still have to spend 3 to 4 hours a week at the house checking condition, replacing supplies, and doing minor repairs.  

What’s got me considering these systems is due to the fact that the under slab ductwork on the downstairs units is rusted out and we were going to be installing a new overhead supply duct system the would require a bunch of soffits to be constructed to conceal all the new ductwork.

I have a business degree (supply chain management and marketing) and have been self-educating on real estate topics, but I'm about ready to get my hands dirty. 

What is there issue supplying the rent roll?

(c) A water supply approved under applicable law, which is:(1) Under the control of the tenant or landlord and is capable of producing hot and cold running water;(2) Furnished to appropriate fixtures; and(3) Connected to a sewage disposal system approved under applicable law and maintained in good working order to the extent that the system can be controlled by the landlord.

To get an accurate answer to your specific region call up some property managers and ask for proposals for their services and as a apart of that have them supply a recommended unit mix based on your market. 

Looks like it, supply at 2.2 months and declining.https://www.bizjournals.com/phoenix/news/2017/05/25/phoenix-tops-us-in-population-growth-more-than-la.html

However, they neglect to mention in order to beat competitors prices, you must supply them with a truth in lending statement/estimate.

As the Boomers retire and downsize, I expect a marked influx of supply of this very type of traditional, single family house.