As my titan is my secondary, this is how I imagine Titan's greet each other, like they greet everything else, with their fists.
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I was playing crucible with my Titan and decided to test out the shoulder charge perk. About halfway through the game I was running through a hallway and saw another Titan. We both activated shoulder charge and we both died. Both our bodies went flying to opposite sides on the room. I laughed so hard I scared my dog out of the room. I later got a message saying "gg"