It seems that the Dynamixel XL-320 is burnt. As you said you applied 12V instead of the recommended ~7.4V.
Could you describe how you were able to connect an XL-320 with an XL-430 ? The connector are not the same and it’s not possible (unless you create your own connectors) to connect the 2 motors together.
Also, please issue a “Technical Support” ticket here, explaining what parts were missing and what is broken, so we can get back to you quickly with a solution. Thanks!