Never borrow significant amounts of money from a private lender without providing collateral security!

It's my understanding that hard money lenders aren't necessarily the formal businesses you might find online, but simply any person/entity willing to provide a short term loan, collateralized by property, and willing to take (mostly) interest only payments and defer full / majority repayment of the principal upon the term end.

Im willing to use my car as extra collateral on the home, and have 20% to put down.

The only reason it is structured as unsecured is if 1) the borrower's net worth is substantial and it's basically a credit card or 2) collateralize funds cannot be secured for a specific reason (such as high volume inventory purchase/sales) but the ave balance stays constant.

I have thought about even putting my cars up as collateral.

However, secured creditors such as a mortgage lender can seek the collateral to satisfy the debt.

My guess is that both properties would be used as collateral since they share the same deed and both you and your in-laws could have to qualify for the loan but this isn't my area of expertise. 

What is the best strategy in using my land(debt free) as collateral for my first duplex/triplex opportunity.

The property would already be the collateral but they also on top of that want a personal guarantee.  

You might find a private lender, hard money lender or commercial lender to lend to you, but they are likely going to want collateral beyond just the property being mortgaged.