Nothing guarantees you against losses from bad investment decisions! That’s the first thing to know.
But all major brokerage firms protect your account up to $500,000 in case the firm defaults. If you are over that amount (check tomake sure your firm is SIPC insured), then I would definitely divide my account in two places.
And you might also want o ask how much you paid in fees and commissions and “maintenance charges” last year. Brokerage firms have some of the highest costs!