So, when you say they not do report on your personal credit report I guess you are talking about non-recourse loans? 

Ana,Based on experience and this market me personally I would avoid a HELOC for a few reasons.

Don't take it personally. 

Fingers crossed in 7 years I have not had to test my coverage, but in the area of my rentals a golf cart is expected with the rental.When they crash into someone else, did that person sign the waiver too ?

My PM used to have a person who did the payments and every year around Feb, she'd send out an annual statement for your taxes showing how much you got in rent, spent in repairs and management fees. 

Note ADU addition in a MF zoned area may be comped with a duplex but I personally think this is wrong as a SFH with ADU is not a duplex.- it is cheaper to buy a property with an existing ADU than adding one   In addition you would receive RE purchase financing.  

What happens if one of you decides to buy a personal home and need the credit to qualify for a new loan so needs to sell this one to make it happen, but the other partner not ready to sell yet, or doesn't want the capital gains. 

Lastly, our DTI is maxed out per every lender we've spoken to recently as all the properties are mortgaged in our personal name.

The total cost of project is more than we can qualify for based on personal income, although our W2 income and credit score all work in our favor.

I would personalize it with a picture of yourself and sign the bottom to make it official.