You should ensure that your broker is a member of the Securities Investor Protection Corporation (SIPC). SIPC protects the cash and securities in your brokerage account much like the Federal Deposit Insurance Corporation (FDIC) protects bank deposits. Securities are protected with a limit of $500,000 USD. Cash is protected with a limit of $250,000 USD. It should be noted that SIPC does not protect investors against loss of value or bad advice.
As far as having multiple brokerage accounts for security, I personally don’t think it’s necessary to have multiple accounts for that reason. Depending on account or transaction fees, it might not hurt to have multiple accounts. It can actually be beneficial to have multiple accounts so long as each account serves a purpose in your overall financial plan. For example, I have three brokerage accounts, each of which serves a specific purpose. One provides low cost stock and bond transactions, another provides superior market data, and the third provides low cost mutual fund transactions.
If you’re worried about asset security, there are a few things you can do to protect yourself. I would recommend you begin by consulting a qualified financial advisor about your risk profile. You stated that a considerable portion of your total assets are in securities. Depending on your risk profile and the amount of your net worth held in securities, you might be better served by moving your money into lower risk asset classes.
I’m not an attorney or a financial advisor. This is not legal advice or financial advice. You can and should consult your own attorney and financial advisor.