Hypothetical question; asking for a friend.
What are the prospects of purchasing, say, a triplex using the equity from another duplex as collateral?
@Colin O'Neill - I think it depends on the type of loan you use. Some commercial lenders may let you do this via them placing a second mortgage on the duplex, but I've not attempted this so not sure what sort of appetite there is for this kind of loan.
If you're going with a commercial lender, just ask them what they can do and they'll tell you. If you'd be using a conventional (i.e. 30 yr) loan, then I don't believe you can do this directly, or at least it would be more hassle than it's worth. Instead, you could either refinance the duplex and pull some cash out, or you could see about getting a HELOC on it. Once completed, you could then use the additional cash towards down payment on the triplex.