How to interact with difficult supervisors after graduation?
Sounds like their issue is that they don't want to spend more time on your defense. They don't have an issue with talking to you in general. So just prepare your defense without asking for help from the supervisors which don't want to help. After you graduate, feel free to write back as you normally would. Just not about the defense.

Just as a remark, I don't get how your other supervisor wants to help you but your main supervisor doesn't want you to ask his help. Presumably the other guy is a free adult, and can make his own decisions. If he agrees to help you, why would your main supervisor object? It's not taking any of his time, and if your defense is better and makes you more likely to graduate, that's good for him too.