cash is typically king in freedom and flexibility to take advantage of buying opportunities so I always go with 20% down to keep my options open.

A duplex policy will rarely have any flexibility in pricing.  

Students coming out of college are saddled by student debt now more than ever, to exacerbate the situation younger millennials are now more than ever placing a higher priority on flexibility in the work place and their living situations leaving many of them choosing to rent over seeking out home ownership; This paired with boomers looking to divest their investments into a more "hands off" approach helps create the perfect storm for young investors ready to take advantage of the multi-family climate.

I’d probably just put them on 2 standard contracts to give you flexibility unless you know the same buyer will take them both.

Portfolio lenders have more flexibility but a little higher rates usually.

Earning a commission on your deals is nice for the extra money, although some choose to waive their commission in exchange for a lower purchase price so that give you some flexibility as well. 

Question 1 - The great thing about partnerships is the flexibility of how to allocate income/loss.

For example:- The market has changed- The project has changed (surprises came up)- My financial situation has changed- My long-term goals have changed- Etc...If something changes, it's always good to be flexible and be able to change with it.  

I know with a $150 - $200 thousand increase on my mortgage I can have more flexibility to get a property to rent and have more cashflow.  

You want to look for the flexibility to not be subject to Co-Insurance requirements or minimum earned premiums and varying choices in deductibles and coverage forms.