You're making shit up. Women got the vote without voting for it. Black people got the vote without voting for it. Black people got civil rights while less than 10% of them were voting. The politicians that passed civil rights were in both parties and it wouldn't have passed without bi-partisan support. Equally, it was both parties that designed, implemented, and sold the war on drugs to incarcerate more black people immediately following the passage of civil rights.
You are 100% correct that the Good Cop behaves materially different from the Bad Cop. The Good Cop doesn't break the law as often, nor as loudly. They don't act beligerently, they don't attack you. What the Good Cop does is hangs out with the Bad Cop and appreciated how the Bad Cop makes it easier for the Good Cop to get compliance from the prisoner.
Yes, GI Joe and COBRA have different policies, but they're created by the same author.
The Ds and the Rs share the responsibility of managing the USA, and the USA is a EuroCentric white supremacist monocultural genocidal psychotic violent belligerent entirely based on the Papal Doctrine of Discovery (don't take my word for it, read RBG) and 100% committed to maintaining control over the stolen wealth of millions of trafficked humans, 100% committed to maintaining the system of racial categorization for the purposes of dividing the working class, ad 100% committed that the working class should never organize and overthrow the capitalists.
So yes, they do act differently. Because that's the roles they play in project of managing empire. But whether you vote for one or the other won't make the country less fascist, less psychotic, less violent, less carceral, less mass murdersome. Voting has never stopped or even slowed down the machine.