Should I reach out to a potential PhD supervisor about applying when they have a relationship with my current PhD supervisor?
Your concern is correct: If your prospective supervisor supported your application and adopted you as a PhD student, that might put a significant strain on their relationship to your current supervisor. It seems unlikely that your prospective supervisor would want to risk that. You can talk to them about your idea, but be prepared for a negative response.

Your best bet would be to look for another supervisor who doesn't have any noteworthy relationship to your current one.