That way they are both liable if one moves out or if they break up ect ect.

I think it would be more prudent to rent four-family units to break up the risk. 

For example: If I sold property A for $130,000 and bought it at $100,000, could I break up the $30,000 into property B?

It's the normal cycles of every industry. 1) Lots of players in space2) Many go bankrupt or get bought out3) Industry consolidates...monopolies form...prices go up4) Something happens to break up industry again----Full cycles that have happened: 1) Cable television.

Business breakups involving close relationships (like family) are inherently messy and expensive.

However, if it is more profitable to break up the portfolio and sell them off piece by piece, that should also be considered. 

You will need an expert to untangle things when it is time to break up, might as well be ready.

The tenant and his girlfriend had a breakup and he does not have the money to pay.

When holding properties separately, you had an easy breakup process if it ever came to it.

Some you cannot resolve and that is when you realize the honeymoon is over and you need an expensive divorce attorney and partnership breakups can be more painful that a divorce.What happens when Partner A wants out and Partner B can't get the cash to pay so they dump the property for a flea market price.