An up to 80% LTV heloc could be an option. 

We ended up opting for a heloc but two months later converted the duplex to an str. 

Sometimes you can find a HELOC with no costs but you have to keep it open for a certain amount of time - I'd get clarifiation from wherever you get the HELOC since your plan is to pay it off when you refi.

By having a wealthier partner on your Operating Agreement, you would have something that lenders could verify for liquidity.Or if you would want to take a HELOC out on your primary you could do that, but I'm not aware of anyone that lends money without confirming that the borrower has funds of their own.

Using a HELOC loan for financing.

There are lenders that offer 10% down investment property loans as 1st lien HELOCs.

Try getting a HELOC.

I have also used proceeds of a HELOC to fix up and repair some properties too

Make sure you know the value the ADU will add to the property before building the ADU. 2) the financing on an ADU is typically far worse than for initial investment property acquisition or is often not leveraged (HELOC, cash out refi, etc).

She heard what the HML was doing and had a nice HELOC at under 2% sitting there going unused and she decided she'd use the bank's money (the HELOC) to make the float (5% diff plus the 3pts on every deal.