At United or just in your cockpit? It’s been plainly spoken for 2 airlines now that the FO has no decision making authority regarding RTO. The FO shall not call reject. The FO is merely to point out an observation (engine failure, fire, etc,) but if the CA chooses to continue, CAs authority dictates the CA is correct and you should not overrule.

In every other instance besides reject they make it clear that you may usurp authority if you feel it’s a safety issue. RTO is crystal clear.

Not sure about the consternation about two people on the controls either. You are PF until relieved by the phrase “I have the aircraft.”