But in the two brokers case, you seem to forget your green position on the 2nd broker is yielding a greater profit, and the average return between the 2 will be exactly the same as the return on the average position.
I get your point, yes and no, it’s not that black and white as there are implications of using different brokers which limit your actual take-home. For example, if you’re in the UK using an ISA account with Trading 212 in one tax year which prevents you from having another stocks and shares ISA within the same tax year. Assuming you exit a position in a stock for whatever reason this tax year with profit, in the case of the ISA you pay no tax on the profits, but in the case of your account with the other broker, you will. Your potential upside is thus limited.
In an ideal world, this shouldn’t even be a conversation piece. I’ve used / use a decent number of brokers, including Interactive Brokers who these guys use as an intermediary. No other broker has this kind of risk management policy for what equates to a “cash account” with limits on how much of a stock you can buy.