How do I responsibly scale cash flowing investments?

[Rob Gifford]

I've consumed dozens of BiggerPockets podcasts and blog posts and am stumped on a big question: How do I responsibly scale up a passive portfolio to the level that would replace my income. Let me explain my situation: my wife and I, both in our mid-30's, are pretty well paid and motivated professionals but also want the freedom to engage in work on our own terms as we get older. To me that means achieving something like financial independence in about the next decade. 

Last year, I purchased a solid duplex that's cash flowing a healthy $500 after mortgages and expenses- it's going great and now have the confidence to start to scale up. What I can't figure out is, long term, how to get the funds I would need to scale my portfolio to a point that would replace our income in what I think is a pretty generous time horizon (10 yrs). By my calculations at our current cashflow level, we'd need 20 duplexes to pay our expenses (that's living pretty simply with a family in Boston). Each decent (turnkey) duplex in my investment market (Upstate NY) requires about $80K for initial investment (down payment and reserves), which is a lot more than I, or most people I'm guessing, can save in 6 months. I realize I could dip into equity I have, but interest to service that debt wipes out the cashflow. I don't have a construction backgrounds and am investing out of state so rehabs aren't appealing to me. 

When you read or listen to BP, it seems that getting to financial independence isn't all that rare. How are people getting the capital without wiping out their cashflow? What other strategies should I explore that won't be too risky?