Take a look at section 11D of the
account agreement. They can liquidate one account to make up a deficiency in another or go after your personal assets. There are some old threads here where this has happened. I'm not sure if the liquidation of other accounts is automatic, so it's possible you could get some extra time to do an orderly liquidation.
You could form separate LLCs, but there's still the risk of them piercing the corporate veil. You'd have to pay separate professional data fees and couldn't readily transfer funds between them.