Is that not the whole point!
In the time taken for the Capt to recognise the failure, decide to stop and then take control the chances of a sucessful reject are reduced by a big margin.
Boeing research indicates that the average pilot will take 3 seconds to indentify a failure, decide to stop, and initiate braking. In Capts only rejects you have to add the change of control time on top again.
I'm sorry but i feel that two people flying the aircraft at one time is a very silly idea.
I demonstrate my ability to perform safe rejects as FO in the sim every 6 months, i'm yet to experience a smooth and effective reject from a pilot that has to take control midstream, and i am certain they will never do it quicker than i could have done.