Intimacy and trust
As I said, the intimacy between Tuco and Blondie is what I love the most about their relationship, the easiness with which they bond so tightly - against all odds being so different and well having tried to kill each other. Their intimacy reaches peak level also when they partner up to fight so they are definitely a power couple. And guess which is the shiniest example of this? The triello. Here Blondie and Tuco put their lives into each other’s hands. Blondie doesn’t even look at Sentenza/Angel Eyes, he waits for Tuco’s cue to shoot and looks at him only. He essentially does what he has been doing for the whole film: wait for Tuco to do something. I am sorry if you will be disappointed but Blondie is not the hero of The Good the Bad and the Ugly, it’s Tuco. He always follows Tuco’s action by doing something consequently. Tuco is about to get shot? Blondie saves him. Tuco is about to be hanged? Blondie shoots the rope. Tuco shoots? Blondie finds him. Tuco is the drive of action, the magnet. Blondie follows. Anyway, Blondie looks at Tuco in the triello because he believes in Tuco, relying on their friendship to save both of their lives. Even though Tuco’s gun is empty, ultimately his decision is what determines the result of the triello - allowing Blondie to shoot on his cue and save them both. Whoever Angel Eyes/Sentenza decides to shoot is the same. Tuco shoots first because he is confident Blondie has got his back. The looks they exchange are telling of their silent agreement. Once more they meet and understand each other in silence. Also these looks in my opinion are erotically charged. Two people shutting out the world completely, relying on each other only. Another scene where Blondie shows how little he cares about anything other than Tuco is when he finds him in the bathtub in the bombed town - and the triello reinforces this concept. If this is not mutual trust and devotion I don’t know what is.
Oh wait I know of another ritual - other than a triello - which is meant to seal devotion and trust between two people where they swear each other loyalty in life and death: marriage. So by the end of the triello Tuco and Blondie are married.