In my current airline and in the airlines I have been working before the decision is taken by the Captain and the actions executed by the CM1, which is normally also the captain unless it is a training flight like a command upgrade. Obviously the Capt may become incapacitated anytime and it is therefore the FO duty (CM2 in normal ops) to carry out the RTO. Other than that I have never experienced SOPS where both the Capt and the FO had the authority to stop the takeoff, except in the above mentioned cases or in case the capt is obviously getting it really wrong like the examples that have been given above in other posts (emergency authority).