Nothing inherently wrong with either, but something to be clear on before investing money, since the controllable successes or failures of a property are going to come down to who is actually putting the deal together and who is running it.I would also be certain how the capital is flowing from deal, through primary sponsor, and down to you, the Spark Rental investor.  

Otherwise, you need to put out some money on a market study.  

Do we, for example, sell our current SFR and put all/most of the equity into a down payment on the duplex and have a lower monthly mortgage payment?

I have put together a "packet" for the appraisers before when they come on-site that shares details about the property (rents, square footage, etc..) and I also share a few "target properties" (aka comps) that I modeled after as a "suggestion" to them.

Want to put 40% down.

After putting the numbers in the rental calculator, it cashflows +216 a month. 

Most savvy investor clients we have are putting 25% down or more to get the properties to positively cash flow.

Anyone have thoughts on moving the two thermostats to the hallway, putting them on my phone for monitoring via wifi with a lock of of xx degrees, and including it in the rent?

I put a small amount down on my first two places and got very lucky with market timing post-COVID. 

I want to put 2 more houses on this property which will give each home almost an acre total for each house.