But I have not seen anyone do that, ergo I must assume the market is too small for such things, that borrowers wouldn't be willing to pay such a fee, or perhaps it's against the rules, or some combination of those factors. 

Think of it from this stand point in a years time you'll have two homes worth combined ($180k + $120k) of $300k for just the cost of the increased mortgage amount of a few hundred dollars a month if you're not able to have the first home paid for by your renters. 

To me, it sounds like a rationalization combined with nothing more than a transfer of the problem. 

I think many on here will find the 50% rule is a combination of the 30% rule for people that get class a and b rentals and the people in the d grade properties averaging around 70% if not more. 

But if you're looking to grow the combination of the captured equity plus, large investment needed, and long time without profit might have you looking into the 1031 option like @Soh Tanaka did.

We have decided to combine our efforts and, well crush it!

Hopefully, that fact combined with our low DTI ratios and high credit scores will allow us to perform a refi in this difficult time.  

I'm new to this world but my current thoughts are it will likely be a combination of BRRR and house flipping with a focus on 100K single-family homes in the Tampa and Lakeland areas.

On the ARV and LTV the difference is most lenders will go up to a certain percentage of ARV (e.g. the ARV is 200K so they can lend up to 150K between purchase and rehab combined).

Should they choose they could combine that sale and also perform a 1031 exchange and that would defer the depreciation recapture as @Debbie J.