I like using different fcms incase 1 goes bankrupt or goes down on a platform or if they dont answer the phones. more of a redundancy safety net. i know my positions i can exit them at a different fcm if i want too. Also some brokerages have better option pricing or they have discount agreements on news services and add ons.What's the reason why you trade the same instrument on multiple accounts? Just curious.
Lastly it allows me to put exact same orders in on the platforms and then check time and sales to see which one fills faster or better based.