Now I have my first which is a house hack.

With those kinds of rents ($5,800 between the 2), even with a 9% cap rate, in my opinion those properties should collectively appraise for nearly $500k.

A D&B rating?

My first offer was $35K below their asking price, but they countered to bring my offer up $10K, so now we are at market value.

I will say that during that time, the HELOC interest rates were very low.  

Offer a significant discount for 1 week and 30 day stays (after setting your daily rate to accommodate the price drop with discount).

It is going to be near impossible to cash flow on a property with 100% financing, especially if that partners rates are above conventional rates.

The reason I am explore this is because the current investors would have a higher IRR (internal rate of return) if I leveraged their cash and brought back a higher returns. 

I'd strongly recommend against a single family home and only target properties with at least 2 units otherwise it'll be difficult to cashflow in this high interest rate environment unless you plan on using a large amount of liquidity in the purchase.

Hopefully the first of many