Just because two people are, say physicists or atheists or believers in liberal democracy, doesn't mean they agree about everything (or even much at all).
The past few years make me think that without sufficient common ground, rational debate is impossible:
•Alice: "Rosemary's Baby is the best film ever."
•Bob: "Devil woman! Satan is speaking through you. But you can't fool me!"
•Alice: "My father just died. Everyone, give your dad a hug. You'll miss him when he's gone."
•Bob: "Cry me a river, loser! Cow farts. Cow farts. Hitler was right!"
•Alice: "I didn't vote for Trump, but I am glad he's talking to North Korea"
•Bob: "Of course you are, Russian bot! I hope your handler is happy"
•Alice: "I used to be a cop, and this video is way over the line."
•Bob: "Antifa scum! How much does Soros pay you to write this BLM propaganda?"
•Alice: "Just a reminder that you can get a free vaccination today, if you live in Charleston."
•Bob: "Great, free autism everybody! Bet you'll trot out your MSM 'facts' now. Science is the new religion."
The web is so full it's bursting with "debates" like these. They are not worth having. They do not change anyone's mind. They cause people to dig in. They just further divide society.