Yes, I believe the term is "Cross Collateralization", a few lenders I have worked with do them. 

@Anthony Testino Perhaps there is opportunity to use crypto assets as collateral / getting crypto backed loans - however, fees high and the usual caveats about crypto etc etc but you usually do not have to sell your crypto to borrowCompanies already doing this/or serving as lending platforms :  SALT, Unchained Capital ,BlockFi

In order to get a construction loan, we had to purchase the lot in cash and put that up as collateral on the construction loan.

Another option would be to get private funding thru a family member / friend and they can place a 2nd lein on your duplex as collateral. 

As such, in an effort to satisfy my own curiosity, is it ridiculous for me to consider cross-collateralizing two vehicles to secure a loan and get started with a house-hackable duplex or other small investment property?

'BTW, that collateral you have- I am going to strip and transform it beginning tomorrow.' 

All required from you is a security deposit to waive off the need of a collateral.

You may want to consider selling a stock that has lost value since you acquired it to offset the gain.unfortunately - you are not able to offset the capital gain by re-acquiring it.Did you look to see if you can get a line of credit with the Index fund as collateral?

Is this true everywhere or is it possible to provide a private lender with a 2nd on the mortgaged property as collateral?

My favorite is to negotiate a substitution of collateral from seller on the purchase money mortgage and quick sale the house.