This sounds like you think Trump supporters live at his rallies and are never normally exposed to different politicians. :P
In reality what would most likely happen if Trump sent some of his fervent supporters to crash a Sanders rally is they would cause a ruckus, not listen to anything Sanders tries to say to start a dialogue, get ejected eventually, and then decide getting thrown out just confirms all the shit Trump has talked about Sanders. People tend to double down on their beliefs when confronted with contradictory information. That's why propaganda (and all political messaging is propaganda) is so effective: once you have someone convinced, it's damn hard to change their mind, no iron fist needed.