Let's say there's a .1% chance of it happening. And let's say that if it happened it would cost you $1M. The expected value of that is $1,000 ($1,000,000*.001). What is the cost of just setting up 2 brokerage accounts, programming for 2 brokerage accounts and tracking 2 brokerage accounts and doing the accounting for 2 accounts? If you value your time at just $200/hr then if you spend more than 5 hours on that in total you're wasting your time right? And that's before we add in the probability times cost that the extra complexity of 2 accounts causes you to make an error or take an extra few seconds and miss an opportunity, either of those will chew through your $1,000 pretty fast as I'm sure we've all experienced.
BTW, .1% chance is pretty high. That would mean that on average this happens with 1 out of every 1000 customers of a brokerage. Or in other words, Schwab has 12M customers so this is happening to 12,000 of them? Seems unrealistically high to me, if you extrapolated to the entire industry it would mean this was happening to literally millions of customers. If you used a more realistic number, like .0001% (leading to 12 customers experiencing this as Schwab) then the expected value of your loss, going to the first example, is a whopping $.10.