When two personalities, one a meek family man the other a covert assassin, discover they share the same mind and body, they must find the organisation that merged them together to reverse the process before the increased brain activity overloads their mind.

Very interesting. ?Makes me want to know more and how you may have pulled this concept off. ?It's a little wordy though. ?I will attempt to shorten it: When a meek family man and a covert assassin discover they share the same body, they must force a mysterious organization to reverse it before the increased brain activity overloads their mind.
I also wonder what this mind overload would mean. ?Would it kill him? ?Or make him insane? ?It might be worth noting the stakes.