I think your design is incorrect. In my opionion, you should have 1 port group going to your primary core device, and a second port group going to your second core device.

Why would you want to connect multiple interfaces from the same stak group into the same core switch? Do you really need that much badnwdith into the core over 1 link.

Redundancy would be my watch word of the day - just my opionon.

HTH>