You both are bringing in a similar amount of money, credit, time, ability, motivation...?

You then  live in one apartment and have the ability to have the rent from the other apartments's rent cover your rent and maybe also be cash flowing

E.g. if my total liability risk ability is $X, I would like to have my invested capital into four such parts below the limited partnership.How do other investor go about this?

Almost every county here offers the ability to look up a property by owner name or address, then look at the property tax bill history for that property and whether or not taxes are paid/current.

It involves buying with the ability to do a cash out refi, or Line/Loan of credit, or a source for non-lien able debt.  

You have the ability to provide for yourself in terms of housing, transportation, food, clothing, utilities, entertainment and more because of your employment.  

Terms are really based on many things, the ability of the buyer/homeowner to pay it back, their credit worthiness, credit score, debt to income ratio, payment history of current debts, and job stability/pay.

I think other things come into play like you're comfort level in buying another one, your ability to manage another, your cash flow situation, your reserves, your other income streams (if you have some) etc.My personal approach is that I look for the best deals when I'm comfortable buying something.

Anyone that would, may subject themselves to a litany of regulations regarding ability to pay, benefit to borrower, etc..

This may be your biggest hurdle in your ability to get a traditional loan.