Cash out equity to buy next property

[Susanna J Ask]

For investors looking to buy and hold cash-flowing properties:

Has anyone considered cashing equity out of one property to make a down payment on another?  Seems like doing this would speed up the rate you can acquire more doors and build passive income quicker. 

There's a couple fintech companies looking to swap equity for a share of appreciation but they don't take a cut of the rental income.  An equity "co-investment". I was wondering what people think about this kind of setup?